She's hitting him for any reason that is bad, whether it's him cheating, him lying, etc. But yes, she hits first.Why is the guy being slapped in the first place? Did the guy do anything to deserve it or is she just randomly attacking him? If it's the first then the guy would be wrong, if it's the second then she should not be touching him in the first place. I hope I am making sense here.
What I'm saying is, technically that guy that had a well-derserved slap can hit her back at his full strength, no matter how little pain her hit was to him.
Do you think that's okay?