Just don't give off the impression that you had used drugs in the pass, or engaged in drug abuse with them. It's fine, as long as it has positivily affect you. I had a friend who entered treatment for rehab. It was a very stressful moment for her family, but even for so, since I was the one who told her to seek help.
Having someone you know abuse drugs and go into rehab is a painful experience.
You will need to explain how it affected you, in what sort of ways. IMO, I think it's okay to write about it.