I'm going to tell my friend that his girlfriend cheated in a past relationship and she lied about being a virgin
Why would you hurt your friend by telling him that his girlfriend cheated?
I think he knows that she is bad for him
You are out of line by interfering in someone else's relationship. You should stay out of this situation because it is not your business. You are going to be the one that ends up on the outside because the messenger is frequently the one that people get angry at.
I only want to help him since he's my good friend.
I'm glad you are trying to protect him, but don't you think it's excessive to try to bring turmoil to your friend? If you want to help him, how about just lending an ear whenever he has a problem instead of gossiping rumors?