#1. Your roommate can't get a prescription for marijuana. He can only get a doctor's recommendation for marijuana. I know it's a stupid little detail, but I just wanted to let you know that piece of info.

#2. You'll be fine as long as your friend claims that everything is his, and you had nothing to do with it.

I speak from experience, because I'm in a similar situation. I have my doctor's rec, but my roommate (owner of the house) does not, and I had a couple of plants growing.