Even if she can be a better person one day, it won't be for you, it won't be with you, it won't be because of you. She won't ever admit she was at fault. She won't ever make it okay.
You're friend doesn't want your help. He doesn't want your advice. He "knows" what's wrong and how to fix it. He's going to have to make his own mistakes and learn his own lessons. If he's as good of a friend as you think he is he'll let you know when he's been kicked to the curb and see's it for what it really was.
Don't give up your friends or peer group. Don't respect their feelings. Be polite when you interact but pretend they just don't exist. Don't give either of them the satisfaction of upsetting you. Don't give them the false vindication that you were the one in the wrong by slinking away. They can both go fuck themselves.
Do as I say, not as I did man. Because as I did kind of sucked. Love who she could have been if you have to but the bitch isn't actually that person. Sorry bro.