If you are correct, she is emotionally cheating, this usually ends up in physical cheating and break ups. But be careful! Do not nag to her about it, you have to be better than him, if you start foul mouthing the guy, she will feel offended and automatically side with him, putting you at a disadvantage. Toughen up, make her realize she likes you not only for the games, try to spend more time with her, take her out to a trip, or buy flowers, take her out for dinner, and the next day, when she's in a good mood, tell her you don't like her getting close to other guys, because you want her to be with you, because she is important to you.
Play it how you like, but don't tell her that she's cheating in any way, she might not see it that way, although it's true, most people aren't aware of emotional cheating. So just make yourself feel important to her, loved, and happy, and you have nothing to worry about! :)
