Most Helpful Girl
Obviously your friends aren't very good friends if they're eating your leftovers, so to speak.
She's being vindictive to have control over you, to know that she can piss you off and upset you. Because maybe you'll want her more since you were so mellow about the break up. Maybe she expected you to fight her on it and fight to have a relationship.
Personally, cut her out of your life entirely, get better friends, and find a normal girl who doesn't need to control or manipulate you or date your friends when she's "done" with you.
Most Helpful Guy
You dated a real psycho my friend. Stay away at all costs. If she's that mean to you, she never really cared about you. Don't care about her. Don't be upset. Don't give her the satisfaction. She's not worth your time, or anyone's time if she's going to be like that. If she's going to be like that, she should be single for the rest of her life. I hate people that manipulate others.
Your best bet is to ignore her and your dumb friends. Find your true friends, and be with them. Talking to your parents also helps a bunch. They make you feel a lot better, especially if they talk trash about her :)