She must ascertain the number of prayers of which she is certain is Qadha (the minimum possibility), and only offer those.
If a person does not fast in the month of Ramadhan due to illness and his illness continues till next Ramadhan, it is not obligatory on him/her to observe the qadha of the fasts which he had not observed, *but for each fast he should give one mudd of food like, wheat, barley, bread etc. to poor. *And she can give food of Kaffarah of a few days to one poor person.
As for the few fasts which made an oath for, and now can not complete, the kaffarah for this is setting a slave free, or fully feeding ten indigent persons, or providing them with clothes.