It may not be written that way into the law but, IN FACT, she can hit him as much as she wants and if he even touches her, the police will take him away and he will face criminal charges. He MUST just stand there and be hit with his arms down.
If this is an ongoing thing, I'm sure his lawyer advised him of this previously.
Womens' divorce lawyers will sometimes advise their clients to start an altercation and if the man responds at all, he will be hauled off as a spouse abuser and she will get everything she wants in the divorce, including not allowing him to see the children.
Be warned, guys!