I want my friend to dump his gf
Is there a reason you want them to break up?
She's a good looking girl but I know for sure he's having an affair with their secretary.
It sounds like you should want his girlfriend to break up with him as he's the one cheating. Either way, a painful truth is always better than a comforting lie.
My friend is being pretty stupid, how can he let go of this beauty? She should dump him.
I can understand how frustrating that is to you. It's good to be honest with your friends even if the truth hurts. You could always mention what he is doing to her, so she can confront him.