It is not Permissible to describe a Dead Person as “the forgiven” or “upon whom is mercy”.
All praise be to Allah and may peace and blessings of Allah be upon His servant and His Messenger Muhammad and upon all his family and Companions. As for what follows:
Many announcements have appeared in the newspapers concerning the deaths of some people, as have the notices of bereavement placed by the relatives of the dead, describing the dead therein as “forgiven” or saying “upon whom is mercy” or such like sayings, indicating that he is one of the inhabitants of Paradise.
It is not unknown to any who has knowledge of the religion of Islam and its beliefs that this is one of the thing which is known to none except Allah, and that the belief of Ahlus-Sunnah wal-Jama’ah is that it is not permissible to testify for anyone that he is in Paradise or Hell, except those who are documented in the Qur'an such as Abu Lahab or those whose end was testified to by the Messenger (Sallallahu Alayhi wa Sallam) such as the ten from among the Companions, may Allah be pleased with them, and their like. Or orders such as them, for whom there is evidence of their being forgiven or shown mercy.
Therefore it is incumbent to say instead: “May Allah forgive him” or “May Allah have mercy on him” or similar words of supplication for the dead.
I, ask Allah, the Most Glorified that He guide us all to the Straight Path. And may peace and blessings of Allah be upon our Prophet Muhammad and upon all his family and Companions.
Shaikh Ibn Baz (Rahimahullah)
Reference: Fatawa Islamiyah Volume 1 Page 207