Hi Ben - sorry for your mother's suffering. I had FM for years...I was given a lot of drugs that did nothing for the pain and weakness but made me feel terrible. Eventually, I discovered that I was very low in vitamin D (testing via 25 OH vitamin D).
Once I began using high dose vitamin D3 (cholecalciferol), all my pain went away. I stopped and the pain came back, so I knew it wasn't just placebo effect. That was 5 - 6 years ago and I remain free of FM pain.
Do you think you can get a test for her? It's better to know the baseline levels so you know how aggressive to be with the dosing.
Many people are found to be deficient in vitamin D especially seniors who aren't typically out in the sun enough and even if they are, the skin loses its ability to convert sunshine into the usable D form.
Jackie