Absolutely! Minis are very well suited for working with persons with many physical and other challenges. I've worked with a therapy riding and driving center for severl years and one of our star therapy horses is a mini gelding that my wife and I trained. Minis are so willing and easy going they are ideal for therapy work.
The chalenge is morre in designing a vehicle to accommodate the driver, It has to be strong enough to securely hold him, easy to get in and out of and light enough for the horse to pull. I would think in the situation you describe, none of those issues would be difficult at all. If your friend can get around well enough to get into an easy entry cart and then remove the prothetics for comfort, there should be no problem making a support for his legs in the cart and he's off to enjoy himslf.
Take your time finding a mini that has a good temperament, willingly stands for being put to the cart and has a good way of going and you are good to go. That really describes most minis anyway so that part isn't too difficult.
Last Summer I was asked to find a horse and cart for a 4 year old boy with MD. He has no core body strength and can't hold his hear up by himself, so he has to be in a wheel chair. We determined the horse would be suitable when the youngster was leading him around the arena from the power chair and the horse simply trotted along with him on a loose lead.
In that case, we built a special cart with a ramp to load a small wheelchair right onto the cart, and a seat next to the opening where the chair sat for the mother to sit. She can drive, and the youngster can also drive using a second set of lines. In time, he'll be driving by himself and I look forward to seeing him showing his mini in driving shows.
There really is no limit to what these little horses can do, and I could write pages on what we've done with our minis working with therapy. GO FOR IT! you'll all have nothing but fun We are ocated in SW WI, if you're anyway near, contact me and maybe I can help.