Qadha prayers of parents is obligatory on the eldest son

1399. If a father did not offer some of his obligatory prayers or fasts, after his death, it is obligatory upon his eldest son, to perform its Qadha. If the son cannot do so, he may hire someone to perform them. Even if father was not able to give Qadha, moreover, as per precautionary measures, even if the father left it as a deliberate act of transgression, it is obligatory upon the son to give its Qadha or hire someone.

Qadha prayers

1379. A person who does not offer his daily prayers within time, should offer Qadha prayers, even if he slept during the entire time prescribed for the prayers, or because of intoxication he did not offer his prayers. However, the ladies who have to leave out daily prayers, due to Hayz or Nifas, similarly, if someone was unconscious the entire prescribed time for the prayers unintentionally, do not have to give any Qadha for them.