Taking a drug that gave her intense and persistent diarrhea is not safe for an eighty year old who already had trouble staying hydrated.

It's not pleasant but it is generally considered to be safe in normal doses. To give you an idea of how ubiquitous its use is, it is used on 3 day old kittens and they're always fine. Then again, each patient is different and know one knows her better than her family, and if they feel it poses a risk, then that's fair enough.

Quality of life is also important, and her experience sounds unpleasant, so your concern is justified regardless.