How should we approach this?

If a girl is able to dish it out, she better be able to take it. I have taught my sons that they are never to hit a girl - unless she hits him first. I've also taught them that if she hits him first, they do not need to beat the crap out of her. Just one hit to make clear that he won't take her bullsh*t, and put her in her place. To make her understand that being a girl does not make her immune to getting back what she gives.

I understand why boys are taught not to hit girls (the whole being bigger, stronger, etc.), but I think some people take it too far and end up teaching their girls that boys can never hit back and therefore they can just beat the crap out of boys and get away with it. That's no more right than a boy hitting a girl first.

If she hits once, tell him to talk away, if she continues tell him to deck her back. Are you sure you want to encourage him to strike women?

I'd say talk to him about the whole situation, and deal with the interaction as a whole (not just the punching.)
Other kids saying what they saw is all well & good for establishing who hit first or who did/didn't hit, but what was the interaction? This was an unprovoked (out of nowhere, unexplained & inexplicable) punch in the stomach? Or what?

Talk to him about it.

I also think that addressing how to respond to a physical attack (including now--how do you proceed?) is an appropriate response & helps put to rest the issue of whether or not refraining from hitting back is "doing nothing." If you DO respond in some way, you're NOT "doing nothing." Focusing on how it's OK to hit back in certain circumstances, then grappling with whether or not that idea is even valid, means that you are ignoring other potential responses.

Why assume that "hitting back" is a main way to respond & focus primarily on trying to codify it?

Get one of his sisters to protect him...lol

I was in an abusive relationship where the guy beat the crap out of me on a daily basis, just because he felt like it. While I would never encourage a male to hit a female, there is a difference between him beating her up, and him defending himself. There is often a double standard that seems to make it out like it's ok for a female to be the aggressor & the male is supposed to just stand there & take it, because he's the male. While I don't promote violence of any kind, I def. think any male who is being physically bullied or hit in anyway has every right to defend himself. Is there anyway that you can contact this girls parents? W/ the video evidence would you be able to press charges on her? I'm sorry he's having to deal w/ this! Truly is a messed up & difficult situation & I wish you the best.

He should still walk away. He needs to go to an adult he can trust and let them know what has happened. since you witnessed the girl hitting him, you can call her parents or even call the police. The girl needs to know that she can't just walk up to someone and hit them. Children practice what they see going on around them. This girl is obviously exposed to domestic violence at home and she needs to get some help.