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 ‘alayhi wa sallam, is His Slave and Messenger.
Zakah enjoys a great status in Islam, for it is the third among the pillars of Islam after the testimony of faith and the performance of Prayer. Failure to pay Zakah is a manifest sin; it remains a financial obligation for the one who is religiously required to give it, even after his death. In Al-Mughni, Ibn Qudaamah said:
"[Unpaid] Zakah does not cease to be payable with the death of the owner of the money; it is to be taken from his wealth, even if not so requested in his last will. This is the view held by ‘Ataa’, Al-Hasan, Az-Zuhri, Qataadah, Maalik, Ash-Shaafi‘i, Is-haaq, Abu Thawr, and Ibn Al-Munthir. Al-Awzaa‘i and Al-Layth were of the opinion that such Zakah should be taken from the third of the wealth, preceding any will and not going beyond the third. On the other hand, Ibn Seereen, Ash-Sha‘bi, An-Nakha‘i, Hammaad ibn Abi Sulaymaan, Daawood ibn Abi Hind, Hameed At-Taweel, Al-Muthanna, and Ath-Thawri held that it should not be taken unless the deceased so wills."
This was also the opinion of Hanafi scholars, saying that, if determined by a will, such Zakah should be taken from the third; and if not willed, it becomes void, since it is an act of worship one of whose conditions is the intention. So, it becomes invalid with the death of the one on whom it was obligatory, just like fasting.
Still, we can say that it is a due right that can be validly determined in a will and does not become void with the death of the concerned person, just like debt. It differs with fasting and prayer in that both are physical acts of worship that cannot be requested in a will or performed by deputation.
Accordingly, your father, may Allah have mercy on him and accept his repentance, should have given Zakah for all the years during which he failed to carry out this obligation, thus completing his repentance. Since he did not do so, the heirs should give Zakah on his behalf for all those years.
Allaah Knows best.