I don't know if this relates at all to arthritis, but I'm going to share some shit with you that I saw first hand.
I have smoked almost every day for 10 years now, so of course I am pro pot. We have all heard of the medicinal benefits, and of course we know them to be true, but I had no idea just how true it was. My girlfriend's mom was found to have severe lung cancer a year ago. It was looking pretty fucking bad, this shit was the size of a fist. She was in chemo, and it was making her ill. She couldn't eat or sleep because of the pain, she was literally waisting away. We are talking about a complete goody two shoes type of mom here, one that never smoked pot or anything.
Her dad knew I smoked, so after hitting rock bottom he asked me to get her some weed, saying they were ready to try anything. She was even taking the marinol tablets, which is synthetic weed. I got her an 8th of the best dank I could find, gave her a pipe and showed her how to use it. Within 2 bowls she was hungry as fuck and asked me to get her some dumplings. She continued smoking every day for the next few months of chemo. She began eating and sleeping regularly and gained back most of her weight, and as of her last checkup not long ago, her cancer has shrunk small enough to be considered insignificant.
I know that's a bit long-winded, but from my experience I would say do what you can to get her to at least try it, the shit works man