One thing a lot of folks do with "abused" dogs is tell them it is fine to behave badly since they were abused, instead of correcting the problems so they can be well-socialized happy dogs. Hopefully your girlfriend is working with some professional, or experianced folks to correct the behavior.
I've fostered dogs, and I do NOT tolerate biting. There have been dogs that I kept and took back with the reccomendation that they be put down, and some that were perfect after a few months. Not every dog is salvagable. Sorry, but that's my opinion.
If you were warned the dog was a biter, you should have taken precautions. She should have put precautions into effect.
Fostering is a REAL labor of love to save those dogs that would otherwise have no hope, and I give your girlfriend kudos for her efforts.
If it's a real issue, or if she doesn't have the necasary experiance to keep aggressive dogs, I would reccomend she stick with non-agressive foster dogs. There's usually a LOT of dogs that need foster homes, and she can get one that doesn't have a biting problem, and let the more experianced folks deal with the aggression issues.