My opinion is that she can't be interested if she is seeing this other guy, so i would move on and pretty much avoid her until she's just another person. Reading this thread that's probably not a popular view, but your only responsibility is to yourself, and you don't have to explain **** to anyone if you don't want to.
The only reason she's seeing him is because he was the first to ask her out, it's not a matter of not being interested - I could've had her, now I just have to decide what to do so it still works out between us later on