That would be tough call! I know from what I have read about my state, we have a right under castle doctrine to defend even if it is not our castle! People or property!
But from my son, who was a LEO until recently, the implications of getting in the middle of what might or might not be a domestic situation is not good.
"I had occasion just prior to Xmas, to come upon a GF/BF situation in a vehicle next to mine in a parking lot. I chose of course to not interfer, but if it had escalated into violence, I would have called 911, and re entered the store, We all had come from to let their management know what was going on. I would no way have gotten into that because she would probably have turned on me and then both would be on my skinny self. "