I'm always against my girlfriends friends
Is there something wrong with your girlfriend's friends?
They make her unhappy and she should know it!
I'm sorry your girlfriend is being treated poorly by them, and she shouldn't tolerate that. You need to keep convincing her and protecting her to show her friends are harming her, or else she'll always be beat down.
I'll make sure they don't look down on her anymore!
Revenge might not be the best way to go. You don't have to be friends with anyone you don't like, but maybe your girlfriend doesn't see that she is being treated poorly. Maybe you can all hang out together and point some things out to her.