In short, yes, you can grow at your friends and most likly you will be completely fine. I had my place burglarized and the cops actually recommended I grow at a friends place versus having my plants at my own residence in an appartment building. If you have your recommendation posted with a copy of your drivers license to accompany it your pretty much golden (at least in california you are). Your friend however isn't allowed to touch your plants unless they have a card as well.
However you should do your reading both at NORML and
http://www.safeaccessnow.org/
Here is a blurb i wrote to a friend a while ago that has alil info concerning caregiver rights and shit:
Where I live I can legally posses 6 mature plants and/or (depends on who you ask) 12 plants under 12". Those are California guidelines though. Oakland - a city near me allows 72 mature plants per patient. In addition to plants I am allowed to posses I believe .5 - 2 lbs at my dwelling and 1/2 lb on me at any time. (again also depends on the county and who you ask)
The rules for growing on a larger scale than personal use are a bit wishy washy. If you are a legal caregiver you can grow and posses pot for your patients. However what alot of people don't understand is that a caregiver is more than someone that distributes pot. A caregiver is someone that administers care and has nothing to do with
marijuana except that
marijuana is deemed a medication and caregivers have the right to posses and administer medication to those they give care to. In order to be a legit caregiver the patient must have a problem that requires a caregivers services and you must visit/administer care at least weekly. Many counties also require that caregivers and patients reside in the same county and you must obtain a caregiver certificate from the same county.
Most people don't obdide by these rules and most get away. Some however don't...Eddy Lepp, an american pot god, who I had the fortune of meeting just got sent to prison for I believe a minimum of 10 years for growing with caregiver rights that didn't pan out - legit patients but couldn't prove he gave any other service other than distributing pot.
Another thing to consider is the local police. I live in a very liberal part of california that has had tons of exposure to pot (I live right next to san fransisco). The police that came to my appartment didn't even varrify my recommendation nor asked about plant limits. They even stated that now a days with the laws as they are you can grow about as much as want as long as they dont catch u selling it. They also RECOMMENDED that I grow at a friends who has a private residence - they thought that was much wiser than growing in my appartment. Basically if you're not selling or growing on a huge scale and have your card your golden. Cops are humans also - they don't want trouble or to get you into it and most dont care about pot as long as your not wasting their time over it.
If you want to be completely legal draw up a rental contract with your friend and rent a closet from him for $1 a month. That way anything in the 'closet' is never legally in his possesion. These are technicalities though that will get you off in court however how stop a police for taking your plants and shit at the very beggining. But again - if there is a copy of a recommendation posted the police generally want nothing to do with it. I'ts not worth their time in court to fuck with some guy growing a couple plants when you'll never go to jail.