I think it is awesome that you would support a friend to accomplish such a feat as Mt. Whitney or any other opportunity. I know of a lot of disabled horseback riders and have seen what determination can take on successfully. Many of them have lost limbs and with the help of a trusting horse, are non-stoppable.
Here is an incredible story of one rider:
http://www.gentlehorses.com/Bettina-Eistel.htmAnd here is a success story from Yosemite of a paralyzed vet who took on El Capitan.
http://www.digitaljournal.com/article/274149A good quote (author unknown):
"The person who says it cannot be done should not interrupt the person doing it."
GO FOR IT!...and give us a report from the summit!