Haven't had diarrhea in a long time but I have been battling with constipation. It mostly started after a failed colonoscopy a couple of years ago which was then followed by a bilateral inguinal hernia surgery. I found a special type of fiber that supposedly can help diarrhea as well as constipation. Can't remember which discussion I saw a member post it in but I started using it and it does seem to help for me but it's wasn't easy for me to figure out what amount to use daily. The brand name is Heather's Tummy Fiber - https://www.heatherstummycare.com/c/tummy-fiber/.

If you are thinking it is the same as your mother's condition, Ginger's suggestions in her above response might be the best option.