I'm just going to think out loud here for a moment:
Unless you have a separate power meter for your friend's portion of the grow. Say something like this:
http://www.amazon.com/gp/product/B000RGF29Q?ie=UTF8&tag=dddnc-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B000RGF29Q
You could only figure out a rough cost to produce for him/her. As long as he is only paying you for what it cost to grow, I would imagine you could argue that its within the law. Maybe you already have a number figured out, and you know what is costs you to produce a plant.
Be sure to figure out how much additional time it takes for you to grow his plants. I don't know about you but my time is expensive. However, none of this matters unless you have to go to court. If you do and you can show that you were charging your friend for the cost to grow and not the cost +, I don't see how you are breaking the law.
I can tell you that I do not give away medicine to adults over 21 for nothing, nor does anyone expect that except people that think power and time is free. It would have made way more since to have people grow outdoors without all the security hassles.
The law and anyone who believes that this would happen is ignorant.
Just like Jus Naturale said above, it is being treated like a prescription med. Personally, I am surprised that we can grow our own. Many of the other states are not allowing that. There is no way that the state legislature and politicians are going to give cannabis the same leeway as alcohol. Cannabis is just not widely accepted enough right now. Not many folks are willing to use Google or do any research of any kind because they still will not risk their political career. Even though we have over 50% of the population in the US ready to legalize, there are maybe a handful of politicians willing to get behind it, even though full legalization makes sense on so many levels.
The type of remuneration you receive is between you and your friend.
End rant