Wait until they break up on their own. Do NOT influence it.

Remember when your gf broke up with you? Wasn't it because she found god or something? That's exactly how this guy would feel. And that's not even taking in to consideration his mental disorders. Fuck, I'm on medication for manic depression (same thing as bi-polar) and If I heard that my gf (again, if i had one) dumped me for another guy who she had feeligns for while we were in our relationship, id get insainly depresssed then break out into a blind rage. Mind you this is with the meds, if hes not medicated, AND skitzo and all that other shit, it could possibly destroy his mental stability. Manic depressive peopel aren't always in control of what they do. They act on impulse. He could very well try to go after you.

If this guy is a huge asshole as you say, then the relationship will eventually end on its own. No intervention is needed. Just stay friends with her for now. Very good friends. Just lay off the making out or fucking till her bf is out of her life.