She is afraid she will wet herself and may have little or no control of doing so once she becomes aware of needing to go or even thinking of needing to go. I call it Pavlovs toilet. I think of it, my brain says "go ahead". Not what I want to do at all. Disposable protection may reassure her more than arguing that she just went. Reassure her that it will be OK if she leaks as she is wearing a disposable toilet. More pads/underwear or more arguments? It worked at the end of my Mother's life when the effort to get her out of bed was daunting so I called it a disposable bedpan. Maybe this can help you too. But there is no arguing with dementia, as they just cannot remember even things that happened just seconds ago. They are not being stubborn or doing it TO you. But they still feel shame. The "bathroom time" timer recommended also may be a big help.