Need Advice...

All you can do is be there for her and show her you do care. Really that's the simple key to success. It may lead to further problems down the road, but in the long run it's best. Don't try to get her against her mother or anything either, as if she cares for her mother too.. it will just cause a big war when obviously one of you is in the wrong, and one isn't. Overall, your girlfriend will come around eventually. If she doesn't, make it clear to her before things diminished, that she made a big mistake.