Mother in law drama..what would you do?

Do you want to keep this girlfriend? It is her decision how she handles her mother. If she is willing to let her mom treat her badly, that's her decision. Be nice to the mother. You don't have to like her. Don't get your feelings involved. Think about how being nice to MIL is actually doing your girlfriend a favor and not causing any more drama for her. Be polite and pleasant. No aloofness, no silent treatment. You can do this. You need to do it for your girlfriend. I know it's being fake but you can act fake for her sake, right?