Perhaps it will be unpopular, but I think the "father" is culpable.

Consider this, if the father were asked "would he do anything differently?", my guess is, he would strongly regret his judgement with the gun. It's not a handgun, or a rifle. I doubt any one of us would agree to bring a high powered machine gun into our home and give a key to a 19-year-old.

I wouldn't even give my car to a 19-year-old without a lot of proof that he was stable, healthy, trustworthy, etc.

This dad was wrong, even though he meant well.