how do i tell her!!?

You could ask him if he's still interested... set a date and stuff, and then prove it to her that way.

^Ooooh, i like that idea.

//Ari\\

Well, that kind of sucks. But at least she knows that he's a jerk now... I feel bad for you, having to see him all the time. Don't let him bother you, okay?

Tell her straight out your obligation is to her and your shared friendship - its better than her continuing to fall for this guy and him breaking her heart even more down the track. the truth hurts but be honest and prepared for the consequences.