Quite honestly the OP is doing more than I would do. She is giving this horse a shot at being a pasture ornament if he can be pasture sound barefoot.
I would not do this. I would simply put the horse down if the horse cannot be ridden due to chronic Navicular disease. For me, as much as I love horses, they are large money to just keep around because of love or whatever. A horse had to be useful and sound and able to work and if they could not then either they were sold or put down. Horses are livestock.. they are not My Friend Flicka or Mr. Ed or the Black Stallion.
I commend the OP for giving this a go. I would not. Clearly this owner has more compassion than I would. I would NEVER call her "cruel."
OTOH many would call me cruel for NOT keeping a pasture ornament.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer