is it possible that he can use something like puppy linux like distros (essentially live usbs, where you can make a persistent partition and make changes)? that way you can prepare a usb, and maybe teach them how to plug the usb in, and change the boot order to include usb? it would be slow (for boot times, if you use some faster usb, that would help), but does not require any fancy operation requiring him to open his computer.

Or if opening the computer/laptop and installing ssd/hdd is within scope, then maybe you can prepare a ssd with complette install, and just ship that. give him additional video/text instructions on how to open a computer, and install it. (In case their system supports ssds, then I highly recommend ssds over hdd, as they do not require special packaging, would be much faster, and ssd installations are also easier(detachhing and reattaching sata ribbon is hard imo, where as with something like nvme ssd, it is very close to like plugging in usb, and then just screwing or something similar.)) If their system is old, and does not have nvme/u.2 connectors, i would still recommend ssd over hdd, then you would also have to procure (and ship) an extra enclosure (i think they are called caddys) to fit a ssd in a husk, which after closing looks like a harddisk and connects via sata. these steps are tad bit hard, but if it is within scope, then I do not think their is a better option.