I want to tell my girlfriend I am not going to break up with her and potentially throw a mutual friend under the bus.
That seems awful. You would do that to your friend?
Yeah, he isn't the smartest guy so she should believe me.
You should be honest with your girlfriend and you shouldn't let another person get the blame for something they didn't do. It is not the right way to handle your relationships. Do you not want to be friends with your friend anymore.
No I don't like him and don't want him to be my friend anymore.
Even if you don't want to be friends anymore, you should be honest with your girlfriend. Hiding things from her will not help your relationship improve.