Thanks for sharing your concern.
I can very well understand your mothers age and her present condition of oral health.
Her upper denture fits
well that is due to adequate ridge support there.
The lower one falls due to resorbed ridges.this is a common phenomenon seen with elderly people.
Anyways,do not worry.I would advise you to revisit your dentist
and ask for denture adhesives.
It is an adhesive made especially for application at the inner surface of denture .so that it keeps the denture in place.this is bio compatible and usually causes no allergy
to tissues,but still it is best if it is done under dentist supervision for the first time.
you have to be very cautious before introducing anything new to the tissues at this age.
please mention little more details about her lower ridges.when was her last extraction done?
these things can be well evaluated clinically.if required you can go for a second opinion
too in this case.
Hope it helps.