He’d already been shoved hard from behind, and had a pack of kids surrounding him that could take him down. Taking out the leader of the ferals, who’s most aggressive, was likely the right thing to do to avoid a beat down.
We didn’t see enough video to know everything, so I have no opinion on guy’s possible liability. He’s allowed to meet force with equal force. He may be found to have exceeded that mandate.
A more extreme example would be if he had shot her, obviously, or hit her with a bat. Given his size and age, his hands could be construed as deadly weapons.
I wonder if she got a concussion?