My mother has illness that is preventing her from fasting. He has not been able to fast for the past 15 years now. Will she have to give fidyah for each missed day of fasting for 15 years? If so, then how much?

Yes. She need to give 22.5 (0.75×30) kg's of rice or flour for each year.
This means 22.5 x 15= 337.5 kg's
This could be distributed among poor momen people.