What is the ruling concerning my grandmother who did not observe the waiting period (iddah) after the death of her husband due to her ignorance of the Sharia ruling at that time, and she is now 73 years old and wishes to know what she must do?

Allah the Almighty has made it obligatory for a Muslim woman whose husband has passed away to observe an iddah (waiting period) of four months and ten days. If the iddah period is missed due to ignorance or any other reason, it is not to be made up. If the grandmother was ignorant of the ruling, then there is nothing upon her. But if she knew, then she must repent and seek forgiveness.