Hi Harold,
We have a guy at our local trout fishery who has only one arm, but it doesn't stop him catching some good fish!
He casts, then puts the rod butt in an adapted harness, similar to the ones blue-water fishers use. Instead of a cup for the rod to fit in, it has about a foot of aluminium tubing. The rod butt has the 'double handle' unscrewed, and a longer piece fitted, to fit the aluminium tube. Once the rod is placed in this harness, it leaves his hand free to retrieve the fly, or fight the fish when he gets them. I believe he plays fish 'off the reel', on which the drag must be set high enough to enable him to land the fish with his net.
I haven't watched him change flies yet, but he doesn't seem to have too much trouble in this.
I hope this helps?? And best wishes & tight lines to your friend!

As featured in Trout Fisherman, UK, Jan 2005! I'd still rather be fishin'!!