Read Dr. Timothy McCall's response:
Fibromyalgia syndrome (FMS), a painful and exhausting condition, can be frustrating for both the student and the teacher. Many people with the disease are depressed, often as a result of the limitations on their life that the syndrome imposes. When you're depressed, it can be hard to believe that anything will make a difference, particularly if you've had poor luck on prior attempts.
You can't make any student do something if they don't want to. My strategy would be to be patient, gently encouraging, and nondemanding, offering morsels of practice that might whet her appetite for more. If you can get her to do just a bit at home—perhaps lie over a bolster for a few minutes, or do a minute of conscious breathing sitting at the kitchen table—she could find herself wanting to do more. Positive experiences like this, even if brief, can build faith in the practice. The fact that she continues to come to class suggests that she has at least a little faith already.
With FMS, probably more than with any other condition, you or the student herself needs to carefully evaluate how long and strong the practice should be on any particular day. Too much can lead to a bad flare-up in symptoms, sometimes leaving the student unable to get out of bed the following day, so be careful not to overdo it. Start her on a very gentle practice, and modify it over time based on her reports of how prior sessions went and how she feels that day. If she learns to adapt her practice skillfully, it will improve the quality of her life, and she will want to practice more. Your job is to get her to the point that she figures this out on her own.