Look, I get it... my wife has 1 leg (she wears a prosthetic but cannot have it around saltwater) and also has 8 fingers, one or two of those are 'placeholders'...
She goes with me because (shock here)....she likes being with me.. I know.. shock.. heh.. but my biggest fear is not her fishing, she does not fish, it is her ability to swim.
No amount of harness is going to help her swim better in the event of an accident, AND... may even hamper her ability to swim in the event of an accident. What I would suggest, which we did, is find a local help group (My wife used to be a member of one.. a mentor actually) and get them to help with the logistics first. I am also betting they will come up with a solution.
And, as much as I hate to say it (because I am cheap.. lol).. I would seriously look into an electric reel. They are not as much fun for US, but they are also not near as tiring. She will still get to do a lot of the things she wants to do, and she can do it more. She can choose to not use the electric reel, till she gets tired, and then she can switch. That way she gets to fish longer.
You'll figure it out and she will be out there fishing soon, I have no doubt...expect a lot of photos of the fish she catches too