While I might generally agree with the above comments assuming he is the BG; what if he is a GG and the lady is the BG? That changes all of the calculus, and based on your short scenario, I got the discomforting impression that alternative was never considered? Some of the responses might have resulted in a very sad answer.
What if she was the Perp? What if he was defending himself? What if he was undercover LEO? What if she was a LEO?A woman is being attacked in a public parking lot. You pull your gun to stop the attack.
What if it is a domestic dispute, and you are walking into a wall of whooping trouble?
What precipitated the attack? Did you see what started it? What are the facts? Is he loss prevention and she's a shoplifter? Maybe she's a half-crazed sister that wants to go out with some trashy boy?
How can you make any decision on what level of force to use based on assumptions that appear to have been made?