I don't believe having the documentation will be an issue as its not against the law (in medical states obviously) to posess a card. However, if it's a company policy to pass a drug test, at any time during employment, and you fail, then it won't matter on the legality, as it's a company policy. If your employer says you can't fart at work, and you do, and they have proof, they can follow through with policy. Especially if you live in an "at will" state.
This is correct. I had the same worry about future employers having access to my medical info. I just went last week to get my cert. I asked the doctor on the phone before going and then again in his office. As mentioned due to HIPPA your doctor can't release your medical records without a subpoena from the courts. Now I was asking specifically about if I applied for a federal job for example, and the Doc did make it clear that if you fail a drug test a company does not have to recognize that you are a MMMP especially for a federal job they are going to say no way. Same thing if you work somewhere and they have a clear Drug Policy. My doctor says he has written letters for people though and that that has been acceptable even for some guy who worked for Honda, now of course that's a Japanese company...
Now, I think it goes without saying if you show up to work blown out, or if you are even glassy eyed and all your co-workers know you smoke and your Managers/Company Owner does NOT know and does NOT support their employee being high at work, you are playing with fire.
For me, I don't work somewhere where they drug test, so I was basically checking just incase. The bottom line is: legally your friend's work cannot legally access her medical records- if they have a strict anti drug policy and condut drug tests- she will not be protected unfortunately. But if she already smokes that doesn't really make a difference. If she gets it though I would not share that info with anyone in the workplace, no need to draw attention when essentially nothing will have changed.
Finally, don't just take it from me, have her call the clinic she is going to and talk to her doctor, my doctor had no problem talking to me over the phone before I went for my appointment.
Hope this helps, good luck to your friend!