Assalaamu alaykum. I knew a woman who died some years ago. She was born Muslim but did not live like a Muslim. She used to eat pig, had children from zina (fornication), did not wear the hijab, did not pray, and so on. She was the mom of a sister that I know. Am I allowed to fast some Ramadans that she did not fast when she was alive in order for her for to get the reward?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

This woman has gravely wronged herself; you should strive in supplicating Allaah for her to a good effect and seek forgiveness for her. Hopefully, Allaah, The Exalted, will forgive her and pardon some of her misdeeds if she died as a believer in the Oneness of Allaah. Her relatives should deduct from her estate the expiation money to feed one poor person for each day of fasting that she missed; the minimum amount to be offered is one mudd (750 grams approx.), or it would be better if they give out half a saaʻ, which is about 1.5 kg (of wheat or any local staple food) so as to avoid the difference of opinion among the scholars in this regard. Sharh Al-Iqnaa’ reads:

“If the person delays making up of the missed fast days for no valid excuse and passes away before or after the following month of Ramadaan (without making up for the missed fast days), then his relatives must feed one poor person for each day that he missed." [End of quote]

They may also make up for those missed fast days on his behalf; the Prophet, sallallaahu ʻalayhi wa sallam, said, 'Whoever dies while he still has some fasts to make up for, then his heir should fast on his behalf.’” [Al-Bukhari and Muslim]

As for you making up for the missed fast days on her behalf, if you are not among her relatives, then it is not prescribed for you to do so according to the scholarly view adopted by Islamweb. Her relatives are the ones who should make up for the obligatory fast days that she missed on her behalf. However, if you seek their permission and they agree, then you may fast on her behalf. Please refer to fatwa 117343.

It should be noted, though, that it is better for you to devote the rewards of your acts of worship and fasting to yourself, and you can supplicate Allaah, The Exalted, for her to a good effect and seek forgiveness for her; hopefully, Allaah will bestow His mercy upon her.

Allaah knows best.