Typical battered woman syndrome. As Moosmom said, there's not much you can do but be there to pick up the pieces until she decides she's had enough. She and only she can pick herself up.

The only thing you can do for her is continue to be a great friend for her. I know it's frustrating as hell to watch her go through this, let her cry on your shoulder and do nothing, but she needs as much positive energy and influences around her as possible. Just be careful not to bash him too much or she'll likely turn away from you too, and you'll be left looking like the bad guy until he screws up again.

Good luck!