How do we reconcile the Hadith about the disbeliever being given glad tidings of Hellfire in his grave with the principle of not definitively assigning Hellfire to a specific individual?

It is well-known in Islam that whoever dies as a disbeliever will abide eternally in Hellfire, and Paradise is forbidden to them, as established by texts from the Quran and Sunnah (definition), such as Allah’s saying: (Indeed, he who associates others with Allah – Allah has forbidden him Paradise, and his refuge is the Fire).

While this is the general ruling, we do not definitively assert the fate of a specific disbeliever to Hellfire unless there is a specific text concerning them, because we do not possess knowledge of the unseen. For perhaps they believed secretly before their death.

As for the Hadith (definition) of the Bedouin who asked the Prophet (peace be upon him) about his father, and he replied that his father was in the Fire, and then asked about the Prophet’s own father, to which he replied: (Wherever you pass by the grave of a polytheist, give him the glad tidings of the Fire) – this Hadith is weak due to the disagreement among narrators regarding its chain of transmission. The correct view is that it is a Mursal Hadith.

In summary, we firmly assert that the disbeliever's ultimate destination is Hellfire, based on divine revelation. However, we do not definitively assert this for a specific person unless there is a specific text regarding them.