from where i cheat

hmonkey

you have friend A, a dude, who has a girlfriend.  you have friend B, also a dude, who does not.  A and B do not know each other.

you have a party with lots of friends, and A and B are both there.  B meets A's girlfriend.  B is totally smitten.

since the party, you find out that B has been hanging out a lot with A's girlfriend, especially when A is traveling for work.  A does not know that B has a crush on her.

VarunaTT

I would say something to B and then I would stay out of it.

zsazsa-stl

Ditto Varuna.  Unless B is some sort of predator (and in that case I would hope I wouldn't have invited him to my party), then it is none of A's business.  If A's girlfriend cheats on him, that is between her and A.  Plenty of people are able to be smitten without acting on it.