I'm surprised this has come up. My sensi, who was trained in Aikijutsu for 15 years in Japan (and former Vietnam vet) came across just such a situation. He saw a man really beating up a woman and he naturally did what any extremely capable martial artists does -- he helped the woman and broke the man's arm among other things. However, once he did, the woman stated bitching at him for doing that to her boyfriend. He just decided this was too weird for him and walked away. I'd say if were me, I'll call 911 or the police first and yell from a distance with my hand securely on my gun at the time.