How can my 68-yr-old mother make up for missed prayers & leftover prayers if her health is poor?

My mother aged 68 years praying 5 times prayer from more than 35 years. Since from past 5 years she is also offering her leftover prayers but she does not remember how much she has offered her leftover prayers nor does she know from which age she has started offering 5 times prayer. Nowadays my mother’s health is not so good and she is unable to offer her leftover prayers and sometimes she also misses her current prayer. My question is what she can do for her leftover prayers and for her current prayers if she misses. Kindly guide in details and in English language.