dude he himself has to realize that this girl is a) not only a bitch, b) that she is self absorbed and c) still has feelings for him but just wants it sexually. Your friend probably just had sex with her for the first time, and this makes things harder. Sexual emotions are the worst to get rid of so it may take some time. Be there for him but make sure he understands that you dont want anypart or her in your conversations. Make it clear to her that u want nothing to do with her. If he's truely a friend, then he'll understand and back off a bit. Take her name off of aim so u never have to see it. If you guys go out make sure its not to see her. Over time this problem will go away, especially if he realizes how crappy she is, and without you there, it will make him think twice. Good Luck cause i know many women like her and it takes a good 2 months to get rid of them, unless she finds somebody else.