In my role as a regional manager I sent many people to rehabilitation for drugs, mostly alcohol. All except one came out of rehab a new person with good attitude and much better employee. But over time, usually 6 months to a year they had fallen back into having issues. In the case of drugs vs alcohol it went mostly undetected by their behavior on the job. The primary issue was attendance and dependability. They would just almost disappear for 2-3 days, their manager unable to contact them, etc. Sadly the long term consequences for most was termination and in some cases even death. It is hard to see someone you have known for years fall into such an abyss.
The one things that is an absolute necessity, I think, is they have to stop associating with those with the same behavior. Addiction is not a easy problem to solve. Only the individual can stop the cycle - I think (not an expert).
As far as old friends - I also have a similar situation with two of my oldest friends. In one case I have known this friend for over 45 years. In the meantime he has been through 3 divorces, my wife doesn't like him coming around anymore and the dude is a total narcissist who is constantly blaming others for his problems. He usually calls me when he is drunk and it actually ruins my evening because there is no conversation just blab blab blab, blab. I can't get a word in edgewise and it never fails at the end of our conversation he will always bring up religion. I listen for a while and then just try to politely hang up. But it makes me feel bad and I wish he had never called.