You might be out of luck according to the regulations, but if I was you I would ring the Ryanair helpline number (can't remember what it is) and say you need one. If/when they tell you that you should have booked it (how ?) at the time of booking, point out that at the time of booking the person didn't need a wheelchair, but they do now . . . in other words they had an accident.
This may not be correct but they have no way of knowing this. Obviously there will be people in this situation and Ryanair do have the cheek (in my view) to charge every single passenger a wheelchair tax/charge/subvention/levy - the least they can do is provide the service.
z