My ex is an alcoholic and I do believe that drink and drug addiction changes people; it changed him. And so I also believe they getting clean/ dry can return them to the people they once were if coupled with therapy. I’d be very wary still of letting someone like that back in my life until they’d proved over a long period of time that the new found abstinence was going to stick. Relapses are common.
Did you know him before he was on drugs?