To justify a kick while on the ground she must have said something the equivalent of "I am going to murder your cat while you sleep" and I doubt that is what happened.
To justify the kick, he merely needs a reasonable belief that she was getting up to continue striking him, and that his use of force was intended to stop this continued threat.
She had just regained her balance and she was getting up. The kick knocked her back over, putting her back on the ground.
The kick pissed me off, but it is not unreasonable to believe she was rising to continue her initial attack. Some level of force against her is justifiable under that condition.
