Young adult's behavior at home: At what point would you find it unacceptable and what do you think is a good resolution?

This is in relation to some troubles that a good friend is going through. I am not asking on her behalf, but more as speculation about how I would handle it if I were in the same spot. I am trying to be supportive but I don’t push advice on her. I am just curious what other parents might do or not do to rectify this.

My friend and her husband are in their early 60’s. They have two sons, the youngest is off to college very soon and the oldest is 21, staying home, got fired from his job, and partying. He had some money saved and was getting unemployment. He crashed two cars (which the parents replaced), has people over to drink and smoke pot, and when the parents (my friends) get upset, he throws tantrums. His tantrums have included punching a hole in the wall, breaking glasses, ripping the phone out of the wall, and ranting for hours. Last night, he took my friend’s (his mom’s) keys away. She had to call her brother to come over and get the keys back. The dad (my friend’s husband) is on vacation. The parents can’t vacation together as the kids will have a major party.

Cops were called recently as the oldest was with an underage girl who was drunk in his room. The cops told my friends that there are rumors around town that the son is selling pot (which I speculated because I don’t see how he could be unemployed for so long and still going to concerts and living large).

She (my friend) doesn’t want to call the cops on him because she doesn’t want him arrested, which will hurt his future. She doesn’t want to throw him out because she doesn’t want him in the street.
My friend and her husband are professional people and the kids were not denied much in the way of material goods.

She is going to therapy to sort through her feelings and choices.

I know she is not the first person ever to have a child who is behaving selfishly and “taking over.”

What would you do, or did you do, in this situation?