Certainly sounds like a fiend to me !

I must admit I found myself getting upset at someone offering me an electronic tuner at a session before. He might have taken it as an unsubtle way of telling him he was out of tune and taken offence. Some people believe, rightly or wrongly, that they are better able to tune thier instrument without the aid of electronics. And as I said on another thread recently its pointless being in concert if nobody else is.

Its not clear from your post what the instruments in the session were. If they were all easily tuned instruments then its not unreasonable to suggest that everyone gets into concert pitch, if they want to play together. Music doesn't sound right if everyone is not tuned to the same pitch. If as you say the person joined you, I would have tought the natural thing to do would be for him to tune to you rather than vice versa. Did he expect everyone else to tune to him instead? Or does he think its traditional for everyone to play randomly out of tune?

Its hard to make rules about these things but locally there's a sort of etiquette about these things - If there's a box (accordeon or concertina) people will usually tune to that - its not so easy to tune the box to fit in with everyone else. If its all strings then a 'consensus A' is found, with or without the aid of electronics or tuning forks and everyone tunes to that. Often people will tune to a whistle, but these can be notoriously dodgy for tuning. If there's a piper and no fixed instruments, we'd normally tune to the pipes, but beware because the pipes tend to get a bit sharper as the night progresses.

Of course some people don't bother tuning at all, which can be a real pain. A friend of mine who's a piper tells a story about playing at a session somewhere in the west of Ireland. A guitar player came in and started backing a tune in A minor with chords in 'D'. My friend turned to him after the tune and said "that tune was in A minor" and the guitarist said "Ah sure I never bother with them oul keys meself !"