Assalaamu 'Alaykum wa Rahmatullaahi wa Barakaatuh.
Al Jawaab Billahit-Tawfeeq
If a person had become so old that he/she is unable to fast, or he/she is suffering from an illness which is not expected to be cured, then in lieu of each fast that is missed, one poor person should be given grain equivalent to Sadaqatul Fitr. It is also permissible to feed a poor person morning and evening for one day. This feeding of a poor person in lieu of a missed fast is called Fidyah in the terminolgy of Sharee'ah. It is also permissible to give the poor person the equivalent monetary value of the grains. Hidaayah
The Fidyah for a single fast is equivalent to 1.640 kilograms of wheat flour or the equivalent monetary value. The exact amount should be calculated by the Ulemaa of each era. (Ahsanul Fataawa, Vol.4)
Bear in mind that along with the Qadha fasts that are already pending, she will also have to pay Fidyah for each fast of every Ramdhaan she is unable to observe in future.
Also note that Fidyah may only be given to people who are worthy recepients of Zakaah.