I went to rehab - I told some friends, and did not tell others. Some people still don't know, others will someday, and still more never will. As an addict, it's embarrassing as hell (well, it was for me) and I absolutely HATED talking about it in the beginning, even with my best friends. I'm almost a year sober and still get nervous about it.
As others said, you're a good friend. I am sure you and your pal will have plenty of time to talk this out, when he's ready. Wish more of us addicts had buddies like you!