I have clients with it and know a good number of people with that condition. The one consistent thing I found is that there is no consistency. What works for one person may be wrong for the other. Often a gentle approach with light weights, water and stretching is great, and then you meet somebody who will tell you that only the hard foam roller will do it for them.
I have also noticed that FM is often in the company of anxieties and depression. FM can easily run a person’s life. It is a very multi-faceted condition where one size does not fit all.
The one thing, though, that seems to apply to all is the need to exercise. It is important to encourage clients to listen to their bodies. They may be able to do more one day than another. It requires a great deal of flexibility on the part of the trainer but also understanding and patience.