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
—Quran 9:68
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
—Quran 16:106
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.
—Quran 2:30
فإنه يعلم السر وأخفى
He knows the secret and what is [even] more hidden
—Quran 20:7
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.