I've remember those in Dr's offices and XRay clinics years ago ... a more modest gown, indeed! You get a full cover at the side, instead of that inevitable split down the front or back. And if worse case scenario it comes apart, it's not quite so bad at the side as in the front/back
If you have a pattern that you have used for your Mom for a nightgown or dress, you could convert it over ... you're basically making two sides, and letting it wrap around.