Hi Plain Tired, and welcome to the board! Your user name says it all! I'm new here myself, but I can tell you you're in the right place.
Oh my! I want to see what the experienced people here have to help you. I'm new at this, but my first thought was duct tape!

A nice waist band around the depends... would that prevent her from pulling them down? Of course, if she's built like my MIL, no waist anymore even though she's not heavy, that wouldn't work.
I know these ladies have some suggestions! They're truly a lifeline!
In the meantime, here's corner of a towel to hold onto. Hang on tight! I'm at the other end, and I sure understand! I don't have all the answers, but I'll stay here holding on with you. We have lots of towels! I was given a generous stack when I just recently started on this board!
Blessings,
Emily