Kepler,
There was no way for that guy to know he was a cop, as he was off duty, and not in uniform. It's almost a lock that he had no idea whether the dude had a piece. I am sorry but under those circumstances no way would I have stood and watched that petite young woman get pummeled. At a minimum he should have grabbed something and hit the guy or otherwise distract him. That woman could have been beaten to death.
I would have have grabbed a bottle of Absolut, splashed some vodka in his face, and then whacked him over the head with the bottle.