Do you agree or disagree with a man hitting a woman in self defense? (Obviously violence is not the best option.)

Let's say you witness a man and a woman arguing. He stands there and takes her smacking him in the face more than once then throws a punch and hits him. He loses control after a few of her smacks and her punch, hits her back and she falls to the ground. Would you be on her side? Would you walk away, help her, help him, try to diffuse the situation...?

Sorry to anyone who is offended by this.