Most mainline aircraft in the US have what's called an onboard wheelchair -- a folding wheelchair to move passengers who are otherwise completely immobile up and down the aisle. This will be available to her should she need to get to or from the lav and can't on her own. They'll also probably allow her to use the lav with the door open, provided you or someone else is by the door to give her some privacy. A good FA will ask at the beginning of the flight how they can best assist inflight.
-J.