Maybe this is the defender in me speaking, but I have this philosophy: If a woman is going to be as hostile as a man I'm going to treat her like one. If a women takes a swing at me, and they have in the past, they better be ready to take one back. The same for a guy. I don't discriminate based on sex when it comes to violence. Initiation of violence is wrong, but even the monks learned to defend themselves.
The mentality of people on the topic of women being abusive is the same as pitbulls vs small dogs in biting. Men, like pitbulls, cause more damage due to build, force, size etc., but women, like most small dogs, are more frequent to bite or attack. Now which dog is seen as the problem dog? The pitbull. Why? Not because of frequency, but because of magnitude. The problem is that men are often times too machismo driven to report female abuse and stick to their guns, while if a guy hits a female, they more readily report it because it's more socially normative.
On the topic of defending someone else such as the circumstance we witnessed here... who knows what they'd actually do. We can sit and speculate and say that we'd do one thing or the other but in the moment people look at for #1. Assessing a hostile situation is not an easy thing to do, that's why there are people trained in crisis intervention, but is passiveness the answer? No.
Overall, my take:
1) Aggression and hostility taken against myself, is met with defense and willingness to engage in subduing actions against that person. That hostility is then reported and taken seriously until the issue is resolved legally.
2) Aggression and hostility taken against another person (be it male on male, male on female, female on male, or female on female), intervention is some form is always correct. It may not be the right idea to step in and try to break them up, but calling the police, or approaching them in a group to let them know that there is social concern, works wonders most of the time. You'd be amazed at how often people shut down when others make them feel like they are making a spectacle of them selves.