As someone who has worked in the field of domestic violence and currently teaches self-defense and personal safety, I strongly advise anyone witnessing any act of violence, including domestic, to pick up the phone and call the police. Getting involved is certainly a choice you can make, but is quite dangerous. In fact, at least in the US, domestic violence is the most deadly of all calls that police officers get (for the police officer).
Boot on her neck- bullshyt. Why people don't leave, etc... Domestic violence is not just restricted to physical violence. There is always power & control that comes into play where the guy threatens to take the kids, hurt her parents, tell something awful about her at work -- which then may make her lose her job, if he allowed her to have one, or she may quit out of fear -- and she probably got rid of all of her friends b/c he got angry whenever she hung out w/them, so now she doesn't have a support network to go to when she leaves him... Oh, and don't forget about the threats he's making about the kids, and that she has no $$ to support herself b/c he's made sure he's the center of her world so she would feel like she couldn't do it on her own... And then the apologies and flowers start coming...
Do you really want to and feel qualified to get involved w/that whole dynamic? Get them help, but be careful to get too involved.
And the cultural piece and why some men are violent? That's huge. Books and theses written on the subject. In short, people tend to repeat what they've seen if there isn't a positive intervention. Think about all the messages you received about how to be a man. In your house, did people model respect and good, clear communication? In the media, were you shown that women were people to be treated well and w/respect, or bodies to be enjoyed and used? Were you taught as some men here were, that your strength was for helping and not hurting? All these messages are very important, and I am definitely not about to prove how much better I am than others by not being in a violent relationship either as the abuser or the abused. But I will help as many people as I can either avoid entering a violent relationship by providing them w/good education about options in how to communicate and what they can & should expect in a relationship, and help as many people who want a change out as possible.
And, yes, I just scanned the thread that Anna just posted (thanks), when we change the way we address violence against women, we're changing the whole culture in regards to gender. Both Anglo western culture and latin & caribbean cultures condone violence against women. If it didn't, it wouldn't be the huge issue that it is. But don't mistake that b/c the Western world has so many women in suits means that we don't mistreat our women. The US has some of the worst rates of violence against women in the '1st world'. Don't let the culture issue paralyze you from making change. That's just a smokescreen.
Btw, I can't recommend enough the book, The Macho Paradox: Why Some Men Hurt Women & How All Men Can Help by Jackson Katz if you want to look into this issue more.