I'm going to freak out on my friends gf.
Is there a reason for this?
She is the dumbest person I have ever met.
I don't think it would be right of you to freak out your friend's girlfriend. It would probably do nothing except make them angry, and it might affect your friendship.
Oh believe me I know :-)
I don't think intentionally trying to upset them is a good idea. If you don't care for that person then you should just keep your distance.