A couple of years ago I was at WDW with some friends, one woman had a multiple fracture of her ankle, and a cast above her knee, only ten days before the trip. Afterwards she said the trip was one of the best she had gone on.
She would be best off renting an
ECV from an off-site company, as pushing a wheelchair can get very tiresome, especially if you are not used to it. Also it depends on the weight of the passenger and the condition of the pusher(s). Some information for off-site rentals are:
Walker Mobility:
1-888-726-6837
www.walkermobility.com
Care Medical:
http://www.caremedicalequipment.com/
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
I have used Walker a few times. They will ask for a credit card and charge it the day before arrival. They will deliver the ECV to your hotel bell services before your arrival and pick it up there after your departure. Their rates, especially for longer than a week, are much better than Disney's. Also, their scooters are much better.
Almost all transportation at Disney is fully accessible. The only exceptions are some of the smaller boats and a few of the buses which might not have working ramps or lifts. I routinely use an ECV there and can go just about anywhere.
Have her find out who will be removing the cast and have them draw a line with a marker where it will be sawn off. Then have her bring a "Sharpie" type marker pen with her and have the Characters sign the cast. Warn them when signing to not cross the line. After the cast is removed she will have an interesting souvenier.
She can use a bungee cord to help keep crutches or a folded up walker on the EVC or wheelchair. Usually it would be a short walk with the crutches from the wheelchair or EVC. Note that if you tell the people in advance, they can have a "crutch clamp" mounted to the ECV.
Good luck and let us know what finally happens.