my client of nine years has a combination of Fibromyalgia and chronic fatigue. When we started training together she was very weak and often in pain, in particular her neck.
For several years we did Pilates keeping in mind that she is often tired, had no energy, as well as being dizzy. Slow and controlled movements and a frequent break were needed.
We are now strength training, keeping the Pilates principles of slow controlled movements, breathing, and centering in mind.
Standing for extended periods of time is still an issue but we work around it.
Listen to her when she tells you about pain or discomfort, watch her closely, in particular the trigger point areas, and make sure she hydrates frequently with electrolytes during the session.