Your friend asks two questions, one about doctrine and canonization and the other about salvation. You asked for my take and I'll give it to you as scripturally and clearly as I can. Hopefully I've gotten all the typos out.
I. Are prophetic teachings considered canon?
In relation to the first question, "Are the teachings of Presidents of the Church considered canon or just educational material?": while I am comfortable expressing my own opinions, one of the major functions of a prophet is to speak on the Lord's behalf, not necessarily to reveal new and unusual information, but to say it in a way that will touch the heart and enlighten the mind of the hearer. Therefore instead of speaking for myself, I have selected relevant quotations from two past Presidents of the Church (Joseph Fielding Smith and Harold B. Lee), in which they themselves quote the scriptures and other past Presidents to explain this matter.
[I'm not sure whether the italics in the following are in the original, or if the emphasis was added by me when I threw these quotes into my Gmail account a while back.]
Joseph Fielding Smith - It makes no difference what is written or what anyone has said, if what has been said is in conflict with what the Lord has revealed, we can set it aside. My word, and teachings of any other member of the Church, high or low, if they do not square with the revelations, we need not accept them. Let us have this matter clear. We have accepted the four standard works as the measuring yardsticks, or balances, by which we measure every man's doctrine. You cannot accept the books written by the authorities of the Church as standards of doctrine, only in so far as they accord with revealed word in the standard works.
Every man who writes is responsible, not the Church, for what he writes. If Joseph Fielding Smith writes something which is out of harmony with the revelations, then every member of the Church is duty bound to reject it. If he writes that which is in perfect harmony with the revealed word of the Lord, then it should be accepted." (Doctrines of Salvation, v. 3, p.203-204)
Harold B. Lee - If anyone, regardless of his position in the Church, were to advance a doctrine that is not substantiated by the standard Church works, meaning the Bible, the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price, you may know that his statement is merely his private opinion. The only one authorized to bring forth any new doctrine is the President of the Church, who, when he does, will declare it as revelation from God, and it will be so accepted by the Council of the Twelve and sustained by the body of the Church. And if any man speaks a doctrine which contradicts what is in the standard Church works, you may know by that same token that it is false and you are not bound to accept it as truth. (The Teachings of Harold B. Lee, p. 544)
Harold B. Lee – We can know when a man speaks as a prophet. Now, when does a person speak as a prophet? Do you recall that oft-repeated revelation in which the Lord said:
"And, behold, this is an ensample unto all those who were ordained unto this priesthood [and he is talking of General Authorities], whose mission is appointed unto them to go forth-"They shall speak as they are moved upon by the Holy Ghost."And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation." (D&C 68:2-4.)
If upon reading this you find that there are aspects of the standard which rely upon individual interpretation ("how do you know when a prophet is speaking as a prophet?", "was President Lee speaking as a prophet when he said this?"), all I can say is, "Exactly!" There are many things which are true but which are not taught as doctrine, but revealed by the Spirit to an individual as he ponders the scriptures and words of the prophet. Individual study and meditation are a necessary part of the process, including the process of distinguishing where individual interpretation is needed and/or valid. I'm conscious of the irony in this infinite regress: like Hawking's story of the mythical flat earth on the back of a giant turtle carried by other turtles, "It's turtles all the way down!"
This is so when a General Authority is speaking by the power of the Holy Ghost.
Someone has rightly said that it is not to be thought that every word spoken by our leaders is inspired. The Prophet Joseph Smith wrote in his personal diary: "This morning I visited with a brother and sister from Michigan, who thought that 'a prophet is always a prophet'; but I told them that a prophet was a prophet only when he was acting as such" (Teachings of the Prophet Joseph Smith, p. 278.)
It is not to be thought that every word spoken by the General Authorities is inspired, or that they are moved upon by the Holy Ghost in everything they write. I don't care what his position is, if he writes something or speaks something that goes beyond anything that you can find in the standard Church works, unless that one be the prophet, seer, and revelator - please note that one exception - you may immediately say, "Well, that is his own idea." And if he says something that contradicts what is found in the standard Church works, you may know by that same token that it is false, regardless of the position of the man who says it. We can know or have the assurance that they are speaking under inspiration if we so live that we can have a witness that what they are speaking is the word of the Lord. There is only one safety, and that is that we shall live to have the witness to know. President Brigham Young said something to the effect that "the greatest fear I have is that the people of this church will accept what we say as the will of the Lord without first praying about it and getting the witness within their own hearts that what we say is the word of the Lord" (see Discourses of Brigham Young, p. 135).
Clearly everybody is "saved" to a greater or lesser degree through the actions of Jesus Christ, even the sons of Perdition who at least receive a resurrection. What kind of "salvation" someone is talking about needs to be judged from the context, but in the Church we tend to talk mostly about celestial glory because that is the Church's raison d'être, and we also want to see everyone attain their full potential. Elder McConkie would, if pressed, admit that murderers and adulterers will receive a kind of salvation, but they will also be missing out on even more, and that is why he said salvation is only available within the Church.
1.) Those who are (103) "liars, and... adulterers, and whoremongers, and whosoever loves and makes a lie..." (106) "are cast down to hell and suffer the wrath of Almighty God, until the fulness of times, when Christ... shall have perfected his work" and then receive (89) "the glory of the telestial, which surpasses all understanding". They are (109) "as innumerable... as the sand upon the seashore."
2.) The (75) "honorable men of the earth, who were blinded by the craftiness of men", (79) "who are not valiant in the testimony of Jesus... obtain not the crown over the kingdom of our God." They receive even greater happiness than the liars and adulterers do, because (91) "the glory of the terrestrial... excels in all things the glory of the telestial, even in glory, and in power, and in might, and in dominion." They (77) "receive of the presence of the Son, but not of the fulness of the Father."
3.) Those who (51) "received the testimony of Jesus, and believed on his name and were baptized after the manner of his burial... keeping the commandments they might be washed and cleansed from all their sins..." and who (53) "overcome by faith... [and] are just and true" are (54) "the church of the Firstborn" and (62) "shall dwell in the presence of God and his Christ forever and ever." (58-59) "They are gods, even the sons of God—wherefore, all things are theirs," and the glory they receive is (70) "celestial... even the glory of God, the highest of all." Entering into celestial glory requires making covenants, and the reason the Church of Jesus Christ of Latter-day Saints was restored to the earth was to enable men and women to make and keep celestial covenants.
4.) Finally, there are those who "[deny] the Only Begotten Son of the Father, having crucified him unto themselves and put him to an open shame... who shall go away into the lake of fire and brimstone, with the devil and his angels." They are (38) "the only ones who shall not be redeemed in the due time of the Lord, after the sufferings of his wrath." They (D&C 88:32) "shall also be quickened [resurrected]; nevertheless, they shall return again to their own place, to enjoy that which they are willing to receive, because they were not willing to enjoy that which they might have received." Usually in the church they are called the "sons of Perdition", which means "loss", and (D&C 88:24) "he who cannot abide the law of a telestial kingdom cannot abide a telestial glory; therefore he is not meet for a kingdom of glory. Therefore he must abide a kingdom which is not a kingdom of glory."