I'm going to lie to my friend's gf about him cheating
Why would you lie to your friend's gf?
The stupid bitch just deserved it.
While you don't have to like your friends partner, you should lie to them. It will hurt your friend and it will damage your friendship forever.
What? I can't wait to tell the world that I outed him as a cheater. It will be fun.
So now you're going the other way and saying you want to hurt your friend? I don't understand, but I don't feel you are handling this situation well. If your friend is cheating you should tell his gf and that's that.