Yeah, that's a tough call. My (former) best friend, about a year ago, asked me to add him to my plan, so I did. This August we went to Vegas together and things went south REALLY fast. As soon as we got back home, I asked him to get his own account and I kindly released his number back to him with no drama.
Since I purchased the phone with MY debit card (his money, though), on MY account, I had every legal right to leave him with a useless phone AND keep his number of 10+ years (he had it with Sprint but ported it to my account, basically legally giving me ownership of his telephone number). And, trust me, I was tempted to be a complete prick and eff him over like that but, ultimately, I took the high road.
So, just because you trust the person now, and everything looks good, things between people can, and do, change. Your best bet is to have a parent or sibling do it for you. It greatly reduces the likelihood of them screwing you over. Good luck!