I got a friend whose girlfriend screws other guys and he knows it but he's a wimp to dump her!

She's pretty hot and she wants me to do her and says my friend "likes' being a cuckold. I'm really torn because I don't want to hurt him him more. But this girl is the finest piece of booty I've ever laid eyes on. I'm afraid my little brain will win this war. What should I do? have sex with her or not?