It really depends on how much you value that friendship. If you never want to talk to her again and don't mind her talking (true) trash to all of your mutual friends, then go for it. If you value her as a friend then don't answer any more of his calls. She's hurting and you're digging the knife in deeper. Boys come and go but friendships and reputations last. Maybe it was her fault that he broke up with her, but you don't have to rub it in her face. Maybe he is the hottest guy you've had a chance with so far and you don't want to miss the opportunity, but what happens after he dumps you or you dump him? Will you have any friends left to turn to when its over and you're hurting? I mean, I can go either way on this one. I've seen friends do this and realize it was a big mistake and loose friends permanently, but I've also seen just once, one girl swear to never date her best friend's ex when they broke up, and then wait one month and do it anyway. She and the best friend no longer talk but she and the ex just got married two months ago. So in her case it worked out, but then she was already 25. Its up to you, just try to think long term and try not to be blinded by day dreams.