Absolutely. You/she will either be accomodated automatically or asked if you need accomodation.
I used an electric scooter on our last trip and was having terrible back problems. On the morning we rode Haunted Mansion I was in particular pain. I opted to walk up (with my husband's help), but it was clear I was struggling. Without anyone saying anything, the ride was stopped when I got on and then stopped again for me to get off. It's done so seamlessly, too.
At Buzz Lightyear, they asked if I could get on from a moving sidewalk and I said yes, but the CM walked ahead of me and demonstrated where to put my hand to steady myself and how to get on and off securely.
At many rides (TSMM, RSR and SpaceMtn come to mind), you enter into a separate car, on your own time, and the car is moved and added into the mix of the other once you are ready. No slowly down or stopping of the whole line.
Disney knows how to handle their special needs guests!!