I'm so glad the parents were able to pry their son out of the house. I also agree that Al-anon or Nar-anon would help going forward. It's too easy to let love cloud your memory of the past and give another chance to someone who doesn't deserve it. It's the parents who deserve another chance. Also, I strongly advise not keeping secrets, whether it's out of shame, the wish to spare someone pain, or some other reason.
One of my brothers has been a career addict/criminal. My mother took him in when he was released from prison a number of years back. She didn't want us to worry about her, and she was ashamed that she wasn't strong enough to throw him out of her house, so she told my brothers and I that everything was fine. I chose to honor my mother by believing she told me the truth. Bad idea.
At any rate, when Mom passed away in April, 2001, from a combination of COPD and life with my brother, he was living in the house. One brother wanted to throw him out and change the locks. The other one suggested setting the house on fire (it was a twin). The lawyer told us we couldn't throw him out. If my brother didn't leave the house voluntarily he could take the estate to orphan's court (Philadelphia, PA) and hold things up for years. The estate wasn't large, so that could mean that no one got anything.
It took 8 months of listening to him talk about how he couldn't afford rent, which turned into no one would rent to him (couldn't argue about that, I wouldn't rent to him), and who knows what other excuses. Finally, one of my cousins agreed to let him move in. My brother got a small advance on his inheritance and my cousin got a check for three months rent (from my brother's future share of the inheritance.)
This brother may or may not be clean and sober now. I really hope he is. I'm not willing to invest enough of myself to let it matter. He did tell me once, in what really did seem to be a rare moment of honesty, that when he was using he would say or do absolutely anything to get what he wanted/needed. I don't believe he resorted to physical violence, but who knows. Lying, cheating, stealing, emotional blackmail - all fair game.
It sounds like the son in this scenario is pretty far down that path. The parent's have done what they could. It's time to do what's best for everyone and let their son sink or swim on his own. They can't fix him. They can't even help him. It won't really help - and it will probably make things worse.
Kathy