Actually, I was in a similar situation with one of my friends. Granted, it was slightly different because this girl bitched at him because she said his friends were a bad influence and he needed to change and go to church more often and such. Eventually, she dumped him because he wasn't a good enough christian for her I guess.

After that, I tried to convince him that she was a piece of shit who made him unhappy and stole him from his friends a good bit of the time but he wouldn't get over her.

Finally things ended, the way I think I helped the process was to target her. Piss her off enough that hopefully she'll get tired of being associated with you or your friend. You risk possibly pissing off your friend but hopefully in the end he will understand or get over it. That is one hell of a tough situation bro.