Earlier quoted context omitted.
I've been brutally betrayed twice in Diplomacy. I'm still good friends with those people. It's only a game. If you can't separate player's behavior in the context of a game with who they are outside of it, then maybe board games is not the hobby for you, or at least not social/negotiation board games, which are very common in recent years.
Diplomacy sounds unique, though, because it takes so long to play. In shorter games I might betray you and you me before the night is over. Also, maybe the length makes it eel like there's more at stake?
I still stand by my statement. It's just a game. Just because it took a few extra hours than most doesn't mean it should permanently damage friendships. Maybe take a week or two break from everyone to decompress from the absolutely intense and crazy experience, but not permanently damage a friendship. That's the sign of a very weak friendship, I feel.
As for more at stake...what's at stake? A 'yay, I won!'? It's not a poker tournament where the winner is going to take home $100,000+.