Dear Good Boyfriend,
Stay Out of the Family Mess
Just the fact that you are taking on your girlfriend's family issues enough to write to this website shows a level of concern and real love that I'm very touched by. Unfortunately, my advice to you is stay out of the family mess!
Even if you were married, family issues are always sticky, and in-law relations can be even more problematic.
I can speak from some personal experience. I had a girlfriend when I was eighteen whose parents were terrible to her as well. We spent countless hours on the phone and on dates talking about how they didn't trust her, how they belittled her, how they made her doubt herself. I can honestly say that the best thing I did for her was listen be there. In fact, even if you've been married 7 years, as I have, the best thing you can do is listen be there for her.
Women Want You To Listen, Not Solve Problems
Sometimes (pretty often, actually) women talk to men about their problems, and we think they want a solution. In fact, mostly what they're looking for is support and understanding. Being really listened to may even be at the root of her problem with her parents. You can be the ONE PERSON who actually listens to her side of the story and supports her.
You Would Be Resented
My guess is that if you were to confront her parents, not only would they resent you for telling them how to treat their daughter, but she might even have trouble with you stepping in. This is a problem between her and them. She needs to help them see her as an adult and treat her like an adult. It may mean that it's time for her to move out. It might be good, if her parents are open to it, for counseling between her and her parents, so that they really get to hear her side in the more neutral territory of the therapist's office.
I would do my best to be supportive, stay on good terms with her parents, and be there for her to hear all of her feelings about this transitional time in her life with her parents.