I think it is very honorable to stand up for a lady, BUT, sometimes it's best not to get involved in something that really doesn't involve you. Yeah, if you see a girl get smacked in the face don't just sit there and watch, but to fight another guy because he likes your friend and she doesn't like him is just plain retarded. He may be a druggy, psycho, or whatever but it's not your responsibilty to clean up her mess; especially if she is not your girlfriend. Obviously she must have done something with him to make him go bonkers over her...Being that he is "unpredictable", do you really want to get involved in something that could potentially endanger your life over some he said, she said bullshit? Is this "good friend" (the girl) worth getting beat down for?
Again, I think it's great to hear about you standing up for the chick but to get beat down once, and then go back for more beatings for nothing is either 1. really, really stupid, or 2. something else is behind all this and she's more than just a "friend".
I don't know of any man to take a beating just for the hell of it, unless they starred in Fight Club