If she missed these days due to illness and she wasn’t able to make them up due to that illness, there is no feeding or making up for her needed.
But if she was chronically ill and couldn’t fast, you can feed poor people on her behalf.

My mother passed away without fasting some days can I compensate it by giving food to equal no of poor pple or I have to fast for her. If giving food is right how? Pls explain