If you have decided that you deserve better and do not want him in your life anymore--then go for it and tell her. Would you want to be her? Wouldnt you hope that someone would have the balls to tell you if your man was cheating on you with someone else? Also, I think telling her calls him out on the terrible things that he has done both to you and to her and god knows what other women he's hurt. Maybe it will show him that his selfish actions have consequences. Also, in a way, it allows you to get some of your own dignity back--especially if you didn't know about the girl in the first place. You'll send a clear message that you are not going to sit around while some man runs carte blanche through everyones life indulging his selfish whims at the emotional expense of others.
Be prepared though. She will not believe you. You should be able to have some sort of proof exp. emails etc. Even then she still may not believe you. You should also be prepared for the man to never talk to you again. But if your motive was true, and you weren't doing it to break up the relationship so you could have him...or some other sort of revenge...then telling is the right thing to do in my opinion. If everyone had the attitude "it's not my business" when they witness horrible things--what kind of world would we live in?
If they smell different you know they are cheating on you
also if they start scratching in that place alot *omg they had sex with someone esle :-O*
Your girlfriend's cheating
tell him
Yes you should tell her that he is cheating. Because later, it will hurt her if not told.
stalk her
You can tell for sure that your girlfriend is cheating, by catching her on the act with her lover.
No, she will find out eventually. IF you do tell, she might take it negatively and this might ruin your friendship.
Yes, you should tell her that your having feelings for guys, she might be releaved and understand, its better than her catching you in the act
You probably can not tell if she is cheating on you, but you can ask her and look closely at her reaction. Talking to other guys is probably innocent and nothing to get upset about.
One way to try to tell if your girlfriend is cheating is if she's refusing to spend time with you. If she has a new boyfriend she may want to spend less time with you.
You should tell her that it upsets you and if she does not understand then she is not a real girlfriend and if she does not stop than you should break up with her there are a lot more girls in this world you can find
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.
What I would do is tell him that she is cheating and give him some good proof and if he dosent want to believe you then that's his choice. Hey! You warned him!