There's no excuse for that shit. I can see if she was being raped, but if she was engaging in consensual sex, he could have easily just stopped them and asked the guy to leave.
I recall a similar story where some father's best friend was living with him, he comes into his daughter's bedroom and sees his best bud nailin' his teenage daughter.Ends up getting a shotgun, too bad there was no bullets, so he beats the guy with the opposite end.The other dude didn't file charges, good times.