I really really want to add that you really shouldn't hate HER. She didn't do anything wrong really. All she did was hook up with a guy who she was sure liked her. I'm not saying it's right for a woman to date a guy who she knows isn't single, but, well, you've been in her place with this same man! We women need to stop hating on each other and attacking each other and remind ourselves who is REALLY at fault here.
He's the one who did the cheating, he's the one who broke your heart, and he's the one who is nothing but scum. Hate on him, not her.