It is, in my opinion, a sleep disorder, manifest by musculoskeletal pain. There is some evidence it is a lack of certain amino acids in the diet also. They need to go off caffeine, melatonin at night helps some. They need to stay active, yoga, tai chi or something. If they don't keep moving, they will get to where they can't.
have them go here, they need to be a bit proactive, much as I hate the term proactive
National Fibromyalgia Association: