I had a similar experience as an employer. I was interviewing a young man for a technicians job. On his resume it said something about being discharged from previous employment. Mind you this was the young mans self prepared resume. I knew that to go deeper into the cause was on the edge of propriety. However, I did ask the young man if he wanted to to tell me why he was discharged/fired. He didn’t hesitate to tell me it was about being on drugs. He was caught up in the drug movement with students at the University where his dad was a prof. He offered that he no longer was a druggie. I hired the young man as much for his candid disclosure as I saw potential for the job. Never regretted my decision at any time and the young man was still doing a teriffic job after I left the organization. I believe you have to have cognizance of real character in choosing employees. I guess this comes from personal experience as a non conforming youth.
I agree that there are people who went down a bad path and discovered it before dying or being put in prison for life. The example you gave is a wonderful one. However, I would have been even more impressed if the young man offered a drug test right there and then and offered to take one at any point in time in the future with no discussion or argument. That would have sealed it for me that he was truly trying to show his past lifestyle was, well... one of the past. I hope the best for him in the future.