Praise be to Allah.
No part of the religion of Allaah may be forsaken because of differences in time, place or circumstances, whether it be the laws of inheritance or anything else. These rulings apply in all times and in every place, for Arabs and non-Arabs, male and female, rich and poor, nomads or city-dwellers; all of them are equal in this regard. When Allaah mentioned the laws of inheritance in Soorat al-Nisaa’, He said at the end of the aayah (interpretation of the meaning):
“(these fixed shares) are ordained by Allaah. And Allaah is Ever All Knower, All Wise” [al-Nisaa’ 4:11]
Imaam Ibn Katheer said in his commentary on this aayah: this is an obligation which Allaah has enjoined and decreed. Allaah is All-Knowing and All-Wise, Who puts things in their places and gives each person what he deserves according to his own situation. “And Allaah is Ever All‑Knower, All‑Wise” [al-Nisaa’ 4:11]. (al-Tafseer, 1/500).
Then Allaah says, concerning the rulings and amounts of inheritance (interpretation of the meaning):
“These are the limits (set by) Allaah (or ordainments as regards laws of inheritance), and whosoever obeys Allaah and His Messenger (Muhammad) will be admitted to Gardens under which rivers flow (in Paradise), to abide therein, and that will be the great success.And whosoever disobeys Allaah and His Messenger (Muhammad), and transgresses His limits, He will cast him into the Fire, to abide therein; and he shall have a disgraceful torment} [al-Nisaa’ 4:13-14]
In this aayah, Allaah states that not implementing the shar’i rules on inheritance is a transgression of the limits set by Allaah, and that the person who does this will abide in Hell forever and will suffer a disgraceful torment.
You must realize – may Allaah bless you – that knowledge of the inheritance laws is half of knowledge, as some of the scholars said. Ibn Katheer said: Ibn ‘Uyaynah said: the inheritance laws have been called half of knowledge because all people go through this. (al-Tafseer, 2/497).
If you believe that the person who does not pray is a kaafir and apostate – which is the correct view, and Allaah knows best – it is not permissible for a kaafir to inherit anything from a Muslim’s wealth, or for a Muslim to inherit anything from a kaafir’s wealth.
It was reported from Usaamah ibn Zayd (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “A Muslim does not inherit from a kaafir and a kaafir does not inherit from a Muslim.” (Narrated by al-Bukhaari, 6383; Muslim, 1614).
Based on the above, those who do not pray cannot inherit anything from those who pray regularly, and those who pray regularly cannot inherit anything from those who do not pray.
You should find out whether this person who does not pray has forsaken prayer altogether, or if he prays some of the obligatory prayers and neglects others. If the latter is the case, then he is not a kaafir, because the kaafir is the person who does not pray at all.
From the above it will be clear that upholding the limits set by Allaah with regard to inheritance is obligatory in all cases, whether all the family members accept this ruling or only some of them do so, because the ruling of Allaah is fixed and definite. At the same time we should note that everyone who does not accept the ruling of Allaah is a kaafir who cannot inherit from his Muslim relative and his Muslim relative cannot inherit from him, if he dies when he still rejects the ruling of Allaah and is still a kaafir. And Allaah knows best.