I would obtain medical clearance for this client even though I am pretty sure that you will get a sign off without restrictions.
I would not do anything different with her in many ways than you would with other clients. Do assessments as you would with anybody and start there. Clients with FM are all different from one another. A common thread seems to be greater fatigue and the need to take it slower. I have seen some people with FM using the hardest foam roller you can find and others who thought that the soft one was too much.
I see that you are certified with NASM including CES, Applying the OPT model with its entry into the stabilization phase will be just the way to go. After a few weeks, you will have a better feel for the situation. She may be a client who works on stabilization and strength endurance and will be doing well there.
Encourage open communication with her so that she feels comfortable saying when things are too much without feeling like a ‘whiner’.