Keeping the qada fasts of another person
Q: My mother, 58, is diabetic and a high bp patient. Contrary to the doctors advice, she observes the Ramadan fasts, alhamdulilah. However, in her young days, she casually skipped many fasts and accordig to her estimation she has skipped around 200 fasts. Given her present condition (Allah bless her with health and life) she might not be able to makeup for those fasts herself in her life.
I know that a son can observe/makeup fast for their parents after their death. Can I observe those missed fasts for her in her life too? If yes then can I now make niyat for my past nafal fasts to make up for her numbers? If initially its a no then would paying only fidya be enough for making up? Please guide on this matter.