Both Chronic Fatigue and Fibro vary widely from person to person. Have some serious, matter of fact conversations with your mother about what she can and cannot do.
I have fibro and would NEVER survive a day at wdw if I had to use a
scooter or WC. Sitting for that length of time would lock me up and exhaust me. (Instead, I push my DW, who has different mobility issues, in a WC.)
However, I often wish for the ability to sit somewhere cool for awhile or the chance to take a nap (note: many people with fibro get little out of a nap unless it is hours long). I have found that taking a long (1-1.5 hr) lunch break at a CS place can be re-energizing.
We used to try to get the the parks early, take a break and go back in the evenings. I find that I do better to either keep going or stop completely. In other words, when we try to nap and then go out again I don't have the energy. But, on Jan 1st I managed to keep going from 8am-1am. (Yes, it was crazy but we had 1 day left on old passes and didn't want to buy new ones--don't try asking your mother to do this.)
We now either do alternating long days (with a long lunch and dinner) and days off or only visit the parks in the morning.
If your mother doesn't know wdw, talk to her in terms of hiking trips. How many days in a row could she go hiking? How long could she hike for? Wdw is similar except that the ground is harder.
I would also recommend some discussions about footwear changes and quality. My shoes make a BIG difference in how the rest of my joints react.