First off welcome
I'd be a little wary at first, but obviously the interactions that this former patient had with the nurses at the hospital has inspired them to become clean, and change their life around. Even though being wary is a good thing, I think that this person needs all the support they can get, as there might be a huge stigma associated with them, being a former patient and all. Maybe tell them something along the lines of-I'm proud of what you've done to change your life and if there are any difficulties/issues let me know and I'll be there to support you through it.
Just my 2 cents