So she punched him, and he punched back.
Math
Well....legal "self-defense" is a lot different than what most people think. It's the
reasonable force you use that is
necessary to prevent the imminent, or ongoing use of force of someone else.
Decking her probably wasn't a reasonable response to her punch. It is pretty unreasonable to think that a hard blow like that was
necessary to stop her.
What happened here was a case of provocation. He was CLEARLY provoked, and punched her back. And most people punch back when having had a punch thrown at them (whether it landed or not).
On the streets, it's typical a fight. Legally, it's just a case of provocation. Now, if she had been pummeling him, landing some hard blows (or picked up a weapon, like a bottle or something), he probably would have been justified in hitting her back.
Having said that, she definitely assaulted him with the missed punch. And physically assaulted him with the kick.