I think the Rambler hit the nail on the head.
I've been going through the statutes and regs, and haven't found anything explicitly prohibiting having a friend help out with the bills associated with growing for him or her, but the language is vague enough that the wrong cop and wrong prosecutor could do some significant damage. As Rambler said, keep copious notes and paper to cover your ass, but otherwise, you're not clearly violating any laws by growing someone else's plants. Honestly, you could likely factor in payment for yourself of an hourly wage, so long as the paperwork backs it up and the rate is reasonable. Of course, this is just my two cents, without warranty either expressed or implied.