That really is a grey area. Of course it always best to tell someone when such behaviour is taking place but just be sure you have her best interests at heart.
tell him
She looks more at the guy than you. Check her phone.
It would be the best to tell him... It would be more worse when he sees his girlfriend doing something with another guy... You should handle the problem easy, and explain him it on an easy way. Ask him to talk with the girlfriend about it and handle it from there on.
I'm not sure to be honest but if you goto him and tell him you know about him cheating and he wishes to continue to do so then he isn't worth your love and feelings. no male or female who cheats should be in a relationship.
maybe, maybe not.If he spends more time with the girl he is cheating on.
This seems like an issue between you and your girlfriend. If she is worried about you cheating, you should ask her why. Perhaps she is insecure and needs you to tell her that you would not cheat on her.
You shouldn't have done anything with him knowing he has a girlfriend.
then he must be cheating on you if he has a girlfriend? i mean if he likes you and has a gf then he must be cheating and wanting you?
tell his girlfriend
Are you the person they were cheating or was it someone else? If someone was cheating you, I would say I didn't want to be their girlfriend (or boyfriend if you are a guy) anymore and that I saw them cheating. If it is someone else that was being cheated, I would tell the person who is being cheated and tell the person what I saw the person do.
if he is cheating he will act in a different manner
If your girlfriend is cheating on you, you should break up with her. Don't get mad at the other guy because it takes two to do something. You were not on her mind or she wouldn't of done it. Just remember you don't deserve somebody , somebody.deserves you!