If she has dementia, can she even merit by such sacrifices? I would think it's the same thing as letting your children have meat on Friday -- only feeding children meat would be worse than feeding meat to a senile person of advanced age -- because in the case of children, you'd be forming the habit of eating meat on Fridays, and they will soon achieve the age of reason (God willing), whereas someone in the Autumn of life has only the next life to worry about.
Matthew