We recently took my 91-year-old mother on the Veendam, a smaller sister to Volendam, but only used a wheelchair at airports; Mom is not wheelchair bound yet. But we did notice the staff and crew going out of their way to be helpful to people in wheelchairs and the disabled travelers in general.
I'm sure they will be there to help you with whatever Mom feels she is up to doing. Just try to make sure they direct questions to your Mom and not to you, as if she wasn't there; I know this is upsetting to some elderly people.
Make sure you let your Travel agent know that your mother is wheelchair bound. You want to make sure that you are booked into a "handicapped acessible" cabin. The cabin is usually larger also. The shower doors are larger and the entry door is larger for getting into the cabin. Most all of the cruiseships have these cabins, but they are limited at to how many the do have. Also, the cruiseline will need the information on the demensions of the wheelchair prior to your sailing date.
We were on the Ryndam last year, with my mom in a wheelchair. The personnel on the ship were wonderful, they were there at every turn (formal dining room, buffet, portside) to take my mom where she needed to be. You will be amazed at the service. Going through the buffet one would push her, the other carry her tray. You won't be disappointed in the least.