Why you ask well after a year of sneaking into the girls room you would think he would introduce himself formally. but no the father found a STRANGER NAKED on the girls bed so he swung away.
Why should he? Why do you assume that the kid wasn't under the impression that the dad was an asshole and he would treat him like dirt no matter what? Maybe he was avoiding him because his girlfriend asked him to. I think you're being way too presumptuous. Even if none of these things are the case, what gives the father the right to beat the kid over the head? Defending his daughter is a silly claim, since there would be obvious indicators that he was an attacker (i.e., the girl yelling "HELP ME DADDY!").
The father had every right to be angry with the kid, but he was absolutely in the wrong to do any physical harm to him.