Try putting water in the tub about 4 inches deep. Most cats will not go in the water and hopeful will use the litter box again.
Add extra litter to the pan and use the clumping kind. Makes it easier to clean and cats seem to like it better. Some of the cheaper litters stick to the pads of their feet when digging the hole for the poo and covering it up.
Boots our cat, hates the scented litter. He refused to use it. So we add baking soda to the litter to prevent strong smells.
Another litter box might work if put near the other one, if you put it in the bath it might defeat the purpose in trying to get her to stop using the tub.
