If a girl (woman?) hits you, a man, "unprovoked" should you hit her back?

My son, a 6'4" pro volleyball player (Denmark) left a Chicago bar with his girlfriend and a male friend. Standing outside with 20 or more people a woman approached my son and "got in his face" about talking too loudly below her apartment. According to him, he replied, "Why are you singling me out of 20 more people out here and you live above a bar so what do you expect?" He turned his head and turned back and she clocked him in the chops and nose. Bloody nose, split lip, and loose teeth. A lot of blood. Not a love tap. As he describes, she wasn't too lady-like with tattoos and numerous face piercings, creative language and unexpected aggressiveness. Bouncers intevened and an argument ensued between my son's girlfriend and the pugilist. My son's friend stepped in and the woman grabbed his bal*s forcefully and held on tightly. Police arrive and the woman had retreated to her apartment. They said you should've hit her back and left. Son is historically docile.