I just went through 2 retinal re-attachment procedures (same eye) and you are correct that the head positioning is by far the most difficult and frustrating aspect. After several days, my neck was frozen from stiffness, my spine and hips were in pain, and I was getting an occasional cramp in one of my legs. My medical facility discouraged the use of anything but Tylenol for pain for the first several days. I eventually had to take a little Tylenol 3 (codiene) and a sleep aid. Be sure the Dr OK's any pain meds.
The lack of movement combined with pain meds will generally cause severe constipation, so a stool softener might be in order. I nearly had a second medical issue on that last point (only relieved by epson salts).
Best of luck to your mother with her recovery.
John W
If she has to be face down (for macular hole repair), there are some special chairs which can be used to help maintain this position more comfortably, similar to a massage table; her retina specialist might be able to recommend one. If she is being treated for a retinal tear, and has to hold her head to one side or the other in order to position the gas bubble properly, perhaps pillows plus a recliner will be more comfortable.