It's not so much the fiber that Dr. Bernstein has concerns with but the bulk. Filling the stomach too full can create what he terms "the Chinese restaurant effect" where insulin is released in response to the stomach being too distended. One way to get around that is with fiber supplements or I just use ground flax...lots of fiber in a little bit. Keto hot cereals are along the same lines. 12 total grams of carb per 2 scoop serving with 9 of those being from fiber. Two other things that you need for regularity are fat and water so making sure that your mom is getting enough of both of those will help as well.