The OP's question is would you shoot? If I shoot the unarmed man, I would have a lot of explaining to do to the police, the DA and a grand jury. The SJWs and PC critics would use the event as ammo in the gun control debate. More importantly, I would have to endure a lot of public criticism in the court of public opinion for shooting an unarmed black man. Would I want to put my family through all of that for a stranger when the man is beating her up, but not killing her? Protecting my family is easy, protecting a 3rd party is asking a lot more of anyone.
I might be a much better witness, record the confrontation, cooperate with the police as a witness, etc. But I don't think I would shoot. If the man was choking her, suffocating her, or doing something to actually kill her, that could change my response.