before you set yourself up, he won't feel bad, so don't go into the conversation with that as your goal. i'm not trying to bring you down, because i've been in your situation before and i know all you really want is for him to feel like the piece of shit he is, but he won't. no matter what you say, he won't feel bad about what he's doing because...he's a piece of shit.
i remember my ex had the same kind of thing going on, messaging on myspace and stuff with this chick that wanted his nuts just because he was in a band. i lurked his space, i won't lie. when i called him on it, he acted like he didn't know what i was talking about. when i quoted him, he started crying about how it "don't mean nothin' she's just an old friend" and how he "can't live without" me. i said some really nasty things to him, i mean things that would make a normal person want to die...and i did nothing. he just kept playing me, and i let him.
don't let him do you like that. you're better than that, and this little chick doesn't have room to be talking shit on you if she doesn't know you, but she's just being immature, so don't take it to heart. i say ditch the dude, and if she wants him so bad, let her have him, because if he'll do it with her, he'll do it to her...and maybe that will teach her a lesson.