Replying to Is this right?

Well I'm going out with this girl and I really like her a lot. But see the thing is she went out with one of my goodfriends for awhile. Now they're broken up. So far this could be considered as i dunnno ' not right' or something, BUT the thing is, he dumped her for another girl and he cheated on her throught the whole summer. He doesn't exactly know. But i made a reference to just taking her to a dance once and the kid flipped....he has a girlfriend and everything..well anyways what do you guys think?