Was it legal to punch this woman standing her ground in this scenario?
I stopped the video at several key seconds and it's clear that the woman wasn't "standing her ground" she was moving toward him with her arms outstretched and appears to have laid hands on him just before he struck her.

On the other hand, it also appears that he was moving toward her with his right arm raised in a striking position.

There isn't likely to be any way of telling from the video whether she was anticipating his attack or he was anticipating hers.

My take is that they should both be charged as they both looked like they were itching for a fight.