This king of thing happens all the time. But the question here is is you like the guy. I f you both have feelings with each other, the best thing for him to do is to break up with his current girl friend. You don't want to start a feud with another person, especially if it's an ex. That can get real messy.
If you like a guy and he has a girlfriend then you should back off and only try and be his friend.
If the boy has a girlfriend, find a guy that doesn't have a girlfriend.
wait
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He has a girlfriend. So you butt out. It's not complicated.
you use his girlfriend like a ladder and jump over him
Well, its ok for them to hang out. If you trust her, then yes, if not then go with her. Like a double date.
You shouldn't flirt with him if he has a girlfriend.
Spy on him! A wife caught a guy redhanded giving nude photos of himself to his girlfriend.
You simply ask him.
you can tell him, or his girlfriend if you are friends with her
Yes.