It is justifiable to use force or violence or to kill in the defense of another person when it is reasonably apparent that the person attacked could have justifiably used such means himself, and when it is reasonably believed that such intervention is necessary to protect the other person.
I've discussed this with my dad over dinner a few times. A few conclusions we've come to...
Being a third party, you REALLY don't know what's happening. If she's being repeatedly punched in the face then OK, but if she's just struggling with some guys there could be something else going on that you don't realize. Perhaps she's drunk and was trying to drive home and her friends are trying to take her keys from her. Or any number of other hypothetical situations.
Immediately drawing on the guys could get you charged with aggravated assault (a felony) or worse (shot dead) if things aren't really as they appear. We're not cops and it's highly risky to intervene in domestic situations.
Louis Awerbuck related a similar story. When he was working as a motorcycle cop in South Africa he came upon a man beating on a woman in the street. He stopped, tackled the man and began to fight him. Suddenly he felt a sharp blow on the back of the head. It was the woman, who was being attacked just seconds earlier, beating Louis with her boot and yelling for him to get off her man. He complied, got back on his bike, and left the scene.
Dawg told us a story at CCW class about a man who was wrestling with a woman in the parking lot while she was screaming "RAPE!" Turns out she was a drug dealer who was being apprehended by an undercover cop in a sting operation. If I recall correctly someone tried to step in thinking they were doing the right thing but other officers stopped them from intervening.
Don't know. Depends on the situation. Seen COPS too many times to know that getting involved in some numbnutts domestic dispute is going to cause you more problems than to them cause it looks like a majority of the time they get back together within a couple of days, rinse, and repeat.