I'm going based on fucking New York law which is weak compared to other states.
www.nyccriminal attorneys.com/self-defense-criminal-lawyers/ (Delete the space)
New York is not a stand your ground state. It has a duty to retreat law in place
Duty to retreat means that a person must attempt to avoid a violent situation before they respond with force to defend themselves. Most states do not have a duty to retreat when it comes to using nonlethal force in self-defense, but many do have a duty to retreat before a person can legally use lethal force to defend themselves.
So the thing would be was he actively avoiding the situation prior to hitting her? Would his walking backwards constitute retreat under NY law or not because I honestly don't know.
But it's not a clear cut self-defense situation based on where they are and the fact weapons are involved.
Duty to retreat has nothing to do with your assertion that he's only legally allowed to use an amount of force equal to or less than what she is estimated to be able to use, please don't think that you can just talk about something else entirely and I won't notice somehow.