In the situation described, if your mother is fully incapable of fasting due to old age or severe illness, and she has no hope of regaining her health and the ability to fast, you can give fidyah for her missed fasts.
However, if she does somehow regain her health and the ability to fast later on, it will become obligatory for her to perform qaza for the missed fasts, despite the fact that you have already given fidyah.
The amount to be given as fidyah for one fast is equal to the amount given for sadqa al fitr which is 1.75 kg of wheat or money equivalent to the price of 1.75 kg of wheat.