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.
A group of jurists permitted reoffering the funeral prayer on an individual who was buried outside the city up to one month after the burial. However, if he was buried in the same city, the people who did not offer the funeral prayer may offer it on his grave. Ibn Abbaas narrated that the Prophet, sallallaahu ‘alayhi wa sallam, went to the grave which had been newly prepared and then he offered the prayer for the one buried in it and they (i.e. those who were behind him) also prayed and he recited four Takbeerahs. [Al-Bukhari and Muslim].
A black woman who used to maintain the mosque had died. The Prophet, sallallaahu ‘alayhi wa sallam, asked about her and he was informed that she had died. He said: "Why did you not inform me? Show me her grave." So he went to her grave and offered the funeral prayer." [Al-Bukhari and Muslim]
Allaah Knows best.