A person who dies on disbelief will enter hell-fire, and will not be forgiven.
إن الله لا يغفر أن يشرك به
Indeed, Allah does not forgive shirk with Him
— Quran 4:48
إن الذين كفروا وصدوا عن سبيل الله ثم ماتوا وهم كفار فلن يغفر الله لهم
Indeed, those who disbelieved and averted [people] from the path of Allah and then died while they were disbelievers - never will Allah forgive them
— Quran 47:34
إن تستغفر لهم سبعين مرة فلن يغفر الله لهم ذلك بأنهم كفروا بالله ورسوله
If you should ask forgiveness for them seventy times - never will Allah forgive them. That is because they disbelieved in Allah and His Messenger.
— Quran 9:80
وعد الله ... الكفار نار جهنم
Allah has promised ... the disbelievers the fire of Hell
The issue is that you can not know for certain whether a specific individual actually died on disbelief.
Maybe the person was a believer in secret and was forced to hide his religion:
إلا من أكره وقلبه مطمئن بالإيمان
except for one who is forced while his heart is secure in faith
Maybe he repented and accepted Islam shortly before death and you don't know about it, but Allah knows about it.
إني أعلم ما لا تعلمون
Indeed, I know that which you do not know.
فإنه يعلم السر وأخفى
He knows the secret and what is [even] more hidden
Maybe he didn't receive Islam's message and so his judgement is currently suspended:
وما كنا معذبين حتى نبعث رسولا
And never would We punish until We sent a messenger
—Quran 17:15; see What is the fate of someone who died never having heard of Islam?
If it was any of these cases and the person was a believer and you went ahead and declared that he is a dweller of hell, then that would be wrong.
For legal purposes like funeral, inheritance etc. the person will be treated according to his apparent condition.