Hmm....this is a very tricky question.
Logically, it's only fair if a woman is hit if she's hitting a man. It's basically self-defence on the man's part. But the problem I have with this is that any man is more likely to be bigger than the woman.
That means their levels of strength is just not comparable. It would be like a child hitting a teen. Again, this is in general and may not apply to ALL women as some of them work out or are taller or have more muscles etc...
In a relationship, it is really a big red flag if any one of them is physically abusive. But that being said, I think there are other options in dealing with a woman being physical. The man can just restrain her hands or wrists until she calms down rather than hitting her back (I'm not condoning women who hit their partners!).
It's very easy to just give in to our instinct to retaliate but in the end, men will always be physically stronger and that can result into dealing even more damage than intended. More bounderies will be crossed thus making the relationship impossible or near-impossible to save. And that's the biggest loss there.
Two wrongs don't make a right. It might make him feel better at that point but....that may not be necessarily true as time goes on.
"Ladies don't start fights, but they can finish them." -Marie, Aristocats