Is it permissible to cite the story of a friend who died having abandoned prayer and committed sins, and to mention that his likely destiny is the Fire and that he is being tormented in the grave, in order to admonish friends?

The caller should invite people to Allah using the revealed revelation from Allah, both the Quran and the Sunnah, as this has a greater impact on hearts than stories and anecdotes. It is not permissible to accuse a deceased person of being among the people of Hellfire or of being tormented in their grave unless their denial of an obligation is confirmed, for Allah may forgive them or they may have repented. Slandering the dead harms the living, and the Messenger of Allah (peace be upon him) forbade it, saying: "Do not insult the dead, for they have gone on to what they have sent forth."