I have Ehlers-Danlos hypermobility disorder, horrible hips, loose jaw-sockets, fingers that can bend like noodles. and 3 pins holding my left arm in.
Lefty has almost no external rotation , and likes to sublux (partially dislocate) at the slightest touch. Righty I've somehow managed to keep in decent shape. Even BB riflles cause enough of an impact to my shoulder capsules to be not worth extended use.
I purchased a Schofield replica for the ease of 1 handed loading, just tuck under the arm after breaking the gun, and drop the speed loader in. It was a great weapon for Cavalry while controlling the reins with one hand.
For semi automatics, I regularly practice operating the slide of my weapon with my boot heel, the top of my wallet, or an (already abused ) workbench.
Last round fired, action open, drop mag, invert gun, placing firmly between the knees ( you'll find the sweet spot with practice) right arm back to the mag pouch, drop new mag, slap, release slide, ready to rock.
From empty chamber, drop mag, replace as above, whip back, catch the top of the leather sticking from the top of my wallet, boot heel etc. ( I carry a chained, tall wallet)
If you have the strength in your legs, you could probably push forward to rack the slide while still between your knees, but I prefer to have my pistol pointed at the earth for drills. even with snap caps.
Good luck to your friend, Hopefully he's as Ornery as he sounds. People like to over-help people like us... and once you lose an action or ability, it's hell to get it back.