How about reaching out to some guides or local shops? I can see if I can contact Kate and Justin Crump, not sure if they're on the site or not, but I would think guides may have dealt with this issue before.
Years ago I was in MT, Twin Bridges, hanging out at Dan Rooster Leavens lodge with the guides and they had a client Archie who was there for a month! I had dinner with everyone a couple of nights, and smoked cigars with Archie and other clients. Archie would fish 6 days a week, he was about 72 at the time. The guides would sometimes have to carry him to the boat, since he had trouble with walking on the rocks. He only fished 6 days a week, since he had to go to Butte for dialysis once a week.
I'm sure one of the shops on the Deschutes could tell you or knows exactly who to contact! Also the guys at Project Healing Waters would probably know the answer as well.
Good luck and it's very nice of you to take your friend out! Using a chair in the water may work as well, just have to help him get in and out of the boat. I watched some guys help a gentleman fish the Sky at Ben Howard and he was in a wheelchair.. they just put him in the shallow water and helped him out when he needed help!