This was in Seattle few years ago:
http://www.youtube.com/watch?v=LVgP5zH1Kss
more opinion:
http://www.youtube.com/watch?v=tjnJUdBGqzU
The point is made: if you get involved you are taking the risk of being injured yourself (it could escalate and get serious, they could pull a weapon). But these days you also risk a charge of assault from them - especially if you are a guy. In that video, the boyfriend is remarkably controlled, acts as a 'shield' (taking kicks to the head) and pushing some of them away but does not fight. You might do that for your girlfriend, but would you do that for a stranger?
In your case, I am surprised these girls were not arrested? Especially since they had thrown a bottle and the guy had a gash? Great! They'll just do it again sometime.