There are alot of variables.
All I can say is that I will do what feels "right" at the time.
Having experience in domestic disputes and seeing the damage both physically and mentally that takes place, I know that not intervening can be deadly.
I have also marveled at the fact that people refused to get involved when someone was getting pounded into the dirt and it was obviously one sided and even written about it on this forum.
A heated verbal argument is one thing. The use of physical force is another.
No matter, intervening is about as dangerous a situation that one can get in...even for uniformed cops. If one does intervene, they must tread very softly and be very, very aware.