My initial reaction is that very little good can come of allowing someone to sign someone else up for these reminders. The potential for abuse is high, and even in the intended use-case I don't see folks being all that pleased.
noahth: Im assuming people love suspense. A message like this "Hello X, Someone close to you wants you to quit smoking. You'll know who it is after 1 week of you quitting you habit!" should only make them curious rather than being displeased. Worse case, they could simply unsubscribe & forget ALL ABOUT IT! fair enough?