Are there any circumstances where threatening a family member with physical violence is acceptable?
You can beat the crap out of anybody but your kids. Best if you can just shove them down a flight of stairs and make like it was an accident.
ETA: shove the family, not the kids, down the stairs.
Does the son have a job? Does he routinely disrespect their home (as he was doing here)? Does he contribute to the household, or is he just mooching and not pulling his weight? I might come unglued and flip out and threaten to beat the shit out of an adult child who continually left my home completely vulnerable for anyone off the street to waltz in and help themselves to the things I busted my ass to obtain.
If the husband is normally even tempered, I'm guessing there's waaaay more to the story.
But that's just me.
I'll bet your friend's boy remembers to leave the garage door closed next time.
Maybe that's what it took, after "a million times" of asking, to get through to him. He'll get over it, and in the meantime, he won't leave the house vulnerable to theft and who knows what.