Sorry to hear of her injury. Unfortunately, for a 15-yr-old I believe your most likely option is limited to a manual wheelchair. Neither WDW onsite rentals, nor most of the offsite vendors, will rent for anyone under 18 to use. You could check with Randy's Mobility as they sometimes have rented to older teens who use one at home.
I would still advise a wheelchair from an offsite vendor rather than within the parks. This will allow you/her to use the chair around the resorts and to/from transportation. In-park rentals can only be used within the park gates. Info about some offsite vendors is listed in the
disABILITIES FAQs thread pinned near the top of this forum.
Enjoy your vacation!