@Yaarel furthermore, I gave the example of Mordenkainen for a specific reason: He has a current canon status in official D&D (like I said, mad and amnesiac in a certain D&D adventure). If I gloss over that canon plot point entirely and say he has never had such an affliction and never will, am I slighting WotC in any way?
If I am not, what is the difference between that and putting a friend's PC in my campaign world as an NPC in a way the friend didn't expressly allow? If it's my campaign world (separate from the canon of where the friend plays that character), how is that character becoming an NPC a slight to my friend?