Yes, call the cruise line. They may even have units they can "lend" you during the cruise so you don't have to bring your own. I know its not nearly the same, but they have wheelchairs for use on the ship (you have to ask) so they may be able to provide this too if you call ahead of time. If not, they can at least tell you for sure if she can use it on board.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007