I need more context.
Did the son force himself on his daughter? If so, then yes i completely understand him getting violent.
But what should have happened afterwards is a call to the police and his parents.
Were they having CONSENSUAL relations? Then he should have called the parents and told them he's not welcome in his house anymore.
Hitting someone's son doesn't erase the fact that he gave his daughter such little fatherly guidance that she sought attention from boys and felt bold enough to get sexual with a male in his house while he's home.
If he'd seriously harmed or killed the boy, then what? He puts his family in financial jeopardy paying for a lawyer and potentially gets a criminal record, loses his job and possibly his freedom?
Who will his daughter then go to when she needs comfort from the shytshow that his violence led to? EXACTLY.
You need to really, TRULY know yourself before you have children. You need to know how you react under pressure, how easy or hard it is for your emotions to take over in a situation. You need to know how your spouse would react in certain situations too so well that you can seem to be a mind reader. You both need to have the time and patience to recognize the behaviors you have that your children may have inherited.
The same impulsive nature that had him immediately start swinging on this teenager is the same impulsive nature his daughter inherited that allowed her to just say "fukk it" and let her emotions take over when in the same room as her boyfriend. If you aren't a calm, logical person, for the love of yourself and society, use a condom, get your girl an IUD and do not reproduce. Find your fulfilment by giving back to charity, volunteering at a spiritual organization or going into local politics. Don't have children. This isn't like going to Venice on some "I wanna experience it before it's too late" parenthood is a 24/7 job that requires you to be your best self damn near all the time. If you can't do that, step back and let those who can cook.
There are too many jails. There are too many victims of the "justice" system. There are too many crashouts of all ages. It needs to stop. Not everyone is built to raise an adult and that is OKAY. There's no shame in making your money and minding your business. You won't die if you don't go raw until a woman is pregnant.
Like look at this shyt right here suggested right after I made this post:
ENOUGH.