I would try my darndest to avoid fighting a friend "for real." Of course, in my college years, my friends would get into a few 'rasslin confrontations, and after which someone gained superiority, it usually ended in "OK. You win. Let's go get some beer."
There was one incident, though, when one fellow was heavily intoxicated, and someone started feeding him rumors that one of his friends was sleeping with his girlfriend. For some reason, that made him fly into a rage, and he came after his friend with a fury...
He almost got to him, until one other fellow and I ended up tackling him, and pinning him to the ground, until he wore himself out. It wasn't easy, and I certainly couldn't have done it myself. It's amazing how much of a fight a drunk with an adrenaline surge from his anger, can put up.
My friend and I were both training in the martial arts, and had a decent working knowledge of grappling, so we tried our best to keep him subdued without resorting to strikes.
He ended up straining his shoulder (I never knew someone could be so flexible), but in my opinion, that was a small price to pay, considering what could have happened. He's not really a friend of mine anymore, and still blames my friend and I for "popping his shoulder" and ruining his chances at trying to walk-on to one of the school's varsity athletic teams that year. The final straw was when he was threatening to tell the student affairs people that we assaulted him, but when one of my friends who produced a cassette tape recording (he was practicing on his drums) of the incident, that quickly shut him up.
While the above wasn't a case of having to use potentially lethal strikes against a friend, it's still an example of how people who you think are good friends, can go bad. It rarely happens, thankfully, amongst good friends, but it still can.
If a friend posed a very big threat, then yes, I would use potentially lethal strikes if there were no other way. I'd probably hate myself for doing it, but I'd rather be alive to hate myself, than the alternative of not having to worry...