My mother is obsessed with her bowel movements. Mom is 96 and I noticed a couple of years ago she began talking about her bathroom habits.

If she has dementia no amount of explaining that she does not "need" a laxative will help. In HER mind she has either not gone or she feels like she has to go but can't.
After loosing control over so many things life maybe this is one thing that she feels like she can still control to some degree.
There is a Tea called Smooth Move and it does work. Maybe get a package of it and replace the tea bags with another type of tea and tell her this stuff works.
Or get a fish oil capsule and tell her that is a stool softener and give her one between the ones she regularly gets. (Only if the fish oil will not interact with any medications she is taking. )

Bowel obsessions are probably the most common of all with dementia. There is no rhyme or reason to this. When I visited my mom in her facility, at least 20 minutes of the visit was spent listening to graphic descriptions of her bowel movements. Explaining to your mother that she does not need a laxative, she’s fine, she just “went” will not work. It’s good to hear that she is strictly supervised now to prevent her from acting on her obsessions.

You might want to try the “therapeutic fib” and when she claims she needs a laxative, in a very serious and concerned tone of voice, tell her you will call the doctor as soon as you can and talk to him/her about it. Then you will let her know what the doctor said.