[nscale]

I am not advocating pulling and using your weapon in this seituation, however, personnally I would yell and scream at the guy. As already discussed this would get other peoples attention, but it would also possibly draw the BG away from the innocent victum. I would have your girlfriend call 911 so you can concentrate and be aware of the actions of the bad guy and any other bystanders.
I would not approach the guy, instead I would try to get him to come at me. Now I am no cop and I would hope the worst that could happen here is a shouting match between me and the bg, at a distance. If he came my way I would make him chase me around a parked car or otherwise prevent physical contact (if possible) and make every attempt to escape. This would keep him from doing any more harm to the victom until the cops arrive.

Worst case scenerio, you can not escape his advances and must make him stop. At this point you will protect yourself.

Now this may not be the proper thing to do, and I am not sure about the legalities of this approach. What I do know is this. If that was my daughter being abused. I could not stand by and watch as he continued beating her as we waited for the cops.
And the fact is - That is someones Daughter