Welcome to disABILITIES.
Here is a
link to the official Disney website information about touring with disabilities.
They highly recommend that people who have concerns about their endurance or ability to stand in line rent/bring a wheelchair or
ecv. That is very good advice because the distances involved in a WDW vacation are much farther than people realize. Just to go around WS in Epcot is over a mile! And, I've read estimates that the average visitor to WDW walks between 3 and 6 miles per day. With a wheelchair, she would be using the handicapped entrances, but in general, she will be waiting the same amount of time whether the handicapped entrances are used or not.
You can rent a wheelchair or ecv (electric
scooter) at the parks (the Disney link above gives information on how to do this), but those at the parks can't be reserved ahead of time and there is no guarantee that there will be one when you get there (more of a problem with
ecvs than with wheelchairs). They also can only be used in the park where they are rented.
If you are staying at one of the WDW resorts, your hotel may have a wheelchair the can lend (but no reservation and no guarantee). Some people have had good luck with getting one from their resort. Others have said that no wheelchairs were available or the one that was available didn't meet their needs (like extra wide when they needed narrow, or vice versa).
A lot of people choose to rent from one of the off-site companies. Here's a list of the places DIS posters have reported rented from:
Care Medical:
http://www.caremedicalequipment.com/
Phone (407) 856-2273 Toll Free U.S and Canada (800) 741-2282
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
(800)747-0246