All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is an obligation on your mother to fast as long as she is able to fast unless a trustworthy Muslim doctor informs her that the disease which necessitates breaking the fast is still there. Therefore, if such doctor informs her of this, then she is permitted to break the fast but she is obliged to feed a poor person for every day she breaks the fast if this disease is chronic and there is no hope of its cure.
However, if the disease is not chronic and it is hoped that she will be cured, then she is obliged to make up the days she broke the fast whenever she becomes able to fast after Ramadan.
Allaah Knows best.