Not making up for missed days of Sawm till death

Q 6: My grandmother died while still had to make up for some missed days of Sawm (Fast) in Ramadan. However, she did not know exactly how many days she missed. She would break Sawm when she was menstruating, during her postpartum period, or when she was extremely tired. But, she did not make up for these missed days out of ignorance. By the time, she had learned the rulings of the Sawm she was already very old and unable to observe Sawm. She died (may Allah have mercy on her) while still did not make up for these missed days. My mother does not know how many days her mother did not observe Sawm. Will it benefit my grandmother and make up for her missed days of Sawm that my mother observes Sawm on her behalf during the White Days (13th, 14th, and 15th of every Hijri month), along with Mondays and Thursdays, with the intention to dedicate the reward to her mother? Or will it benefit her if my mother makes an estimation of these days and offers Kaffarah (expiation) for them along with observing Sawm?

A: If you are most certain that your grandmother did not make up for her missed Sawm, and (Part No. 9; Page No. 55) she told you so when she became old, then it is permissible for you to observe Sawm on her behalf with the intention of making up for her missed days of Sawm. This should be done after doing your best in discerning the number of the days of Sawm she had missed. Also, you must feed a Miskin (needy person) for each day she had missed, by giving him approximately one kilo and a half of the staple food of your country. This is an expiation for the delay in making up the missed Sawm until the following Ramadan. The Prophet Muhammad (peace be upon him) said: Whoever dies while having missed Sawm (days not fasted), his Waliy (heir) should fast on his behalf. A Wali is a close relative.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.