Everything you propose carries potential risk for both of you. In addition, marijuana is still federally illegal and no proscription will protect against the violation of federal law.
The information provided in this answer does not create an attorney-client relationship and is not considered to be legal advice.
In Massachusetts possession of an ounce or less is decriminalized. That doesn't mean it's legal, you just get a fine instead of jail time and they confiscate the weed. That does not affect the federal laws wherein weed is still illegal and you can go to jail for possessing any amount. You could get in trouble if driving or walking or sitting with her if the cops thought that both of you possessed the weed (constructive possession). If she was caught buying it from an illegal source she could get charged criminally if it was over an ounce. In my opinion, since the dispensaries haven't opened yet she should be very very very careful where she gets her weed from. Having a valid prescription may not be enough to insulate her from criminal charges.
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This is a relatively new area of law and your question is really good. I would imagine where you do have a lawful prescription to possessing the drugs, you may be ok, but I would not risk it. Nor would I have it in my car with others, then the police could potentially charge you with distribution and other passengers with possession/conspiracy. I would consult a lawyer-there are many other issues about this new, ever evolving area of law.
1. No matter the source, the prescription could be raised as a legal defense to a charge of possession of marijuana. And, of course, under an oz is not criminal.
2. See No. 1 above.
3. If you are possessing the marijuana, meaning it is on your person or under your custody or control, then you could be charged with crim possession, if over an oz. If you are not capable of driving safely because of ingestion of marij, you could be charged with OUI.
Best of luck
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