I'm not going to tell my friend that his gf is not the girl he thinks she is and not feeling bad about it
Have you noticed a problem between them?
No, I'm 100% sure, I've got proof she is cheating.
You should help your friend out if you know his gf is cheating. He has the right to know about that.
That's the fun part. The fun is trying to build his hopes up and watching him getting destroyed.
It seems a little cruel to enjoy that. You should tell your friend so that they can avoid being hurt.