I intermittently use a Twiddler (older version). The learning curve is initially steep but fine with practice. It's not cheap but it's not that expensive, and it works for mousing as well: https://www.tekgear.com/twiddler-4-wrap.html
Like OP's relative, I also could not use my dominant arm, nor have my arm in a position that would allow typing one handed on a regular keyboard. The twiddler was the only commercially available option that I was able to find that would allow me to type in this state. So, another recommendation of twiddler, but with the caveat that the original had several warts, and while they appear to be redesigned, they may still suffer from some of them.
The velcro strap to hold it to the hand, combined with the shape of the keyboard, allowed it to shift position while typing making it harder to use. Photos of the current models show they have a different shape now. Maybe this is less of a problem now? They are also wireless now, so there isn't a wire constantly pulling it out of position whenever you move.
The keyboard markings rubbed off completely after only a few weeks of use on my OG twiddler. Hopefully they have worked that manufacturing issue out for the current models.
Separate from that, when I've temporarily injured one hand/arm or the other, typing on a QWERTY keyboard wasn't that slow for me. Especially if I typed all-lowercase. Though my normal typing style has two hands moving around the keyboard a bit; I don't know whether traditional home-row typists would fine one-handed more difficult.
(Just be careful when Web searching about this topic at work, since it's bumping into an old euphemism joke on Reddit.)