Yeah but you just found a dating profile.. that means nothing. How do you know how often she uses it/if she's used it since they've been together/that she doesn't use it to just chat (I do) or that she will actually act on anything. It's like, generally people don't break up with their partners for talking to other people.. and you don't know that anything more than that is going on. I realize that's what you're trying to prove, but this isn't TV, this is real life and I a) don't think it's your place and b) you can do this by yourself and c) I am not going to help you
It sounds like you still don't like her. Trying to seduce her to make her and your friend feel bad sounds like a really good idea.
Also how do you know he's not okay with it....... when he doesn't even know?