Any suggestions on pants for thin, frail mom?

Maybe children's clothes might work for her. Even children's underwear? Or junior sizes. I have no idea if this would work, just trying to think outside the box.

I wouldn't be comfortable with no underwear, but if she is, I don't see a problem with that. Buy a few styles and sizes. If one of them works well, buy a bunch more of that. If not, buy another small batch to try. Notice the different waist styles and leg styles available. The one that works best is not just a matter of style preference but of body shape. I personally only like high-cut briefs. I skip the bikinis and regular briefs and boy cuts -- no matter how cute they are or what a good price they are. I lost a little weight last year and I had to go down a size in panties, or they just didn't feel good.

I think with pull-on pants it is the same process. As soon as you find one that fits comfortably and washes well, go back to that store and buy several more. Bundle up the uncomfortable ones and give them away.

If she is frail and a fall risk, do a Google search for "HipSaver" and see what you think of the "softsweats pants." I buy them for my mom, and they look much nicer than in the picture!  She likes them a lot.  (There are both US and Canadian sites for ordering these, too.) They go down to extra-small in size, plus you can get them in 2 different lengths. They also have different styles of protection padding, to cushion knees, hips, and/or tailbone in case of falling.

Edit:  leggings and long shirts might be a nice option, too.  Good leggings fit snug without feeling tight.  Mom also loved her leggings, and yoga pants as well, before we had to do the hip protector thing.  Also, is it the leg hole of the underwear that bothers her?  'Cause that's always my complaint!  In which case she might like women's boxer briefs instead of regular underwear.  And it's not my thing, but if a person washes their pants after every wear, I don't see why they can't go commando.

For a very thin person the yoga pants might work. A longer top looks nice and stylish with. No pockets in the yoga pants. The stretchy knit underwear might fit her snug enough. For cotton I like the idea of the children’s but it might not be long enough in the stride?
Maybe take a few measurements.