The key is the amount of liquids consumed, in addition to all those herbal teas which, while hard to drink, do actually seem to clean the body very well.
However, if your mother-in-law is otherwise clean (washes her hands regularly, spot-cleans parts of her body that are dirty on a daily basis, and wears clean clothes for example) and is fully responsible for herself in other ways, this issue may not be a problem for her.