What is likely going on is that, during the retina repair surgery
, her eye doctor
probably injected a gas bubble into the eye. This bubble is supposed to act like a "splint", helping to reinforce the retina and keep it in place at the back of the eye while it heals and reattaches.
However, since gas bubbles are subject to the forces of gravity and will tend to rise upwards through the liquid inside the eye, it is important to avoid any position that would cause the bubble to float away from the retina. Specifically, this means no lying on your back, because this would cause to the bubble to move towards the front of the eye where the pupil is. If this occurred, the bubble would no longer be in contact with the retina and eye healing could be interrupted or slowed down.
Your wife should pay close attention to the instructions about what positions she can have her head in for the length of time recommended by her eye doctor. Over time, the retina will heal and the gas bubble will dissipate, at which point she will be able to go back to her normal activities and positions. I recommend that your wife discuss this issue with her doctor to make sure that you have understood these instructions correctly.