What should the relatives of someone who died suddenly do for them first, what is the best prayer for them, and for how long should they continue praying for them?

It is an honor to the deceased to hasten their burial after verifying their death. If there is doubt about their death, one should wait until the signs of death become clear. There is no specific supplication to be made for the deceased immediately; rather, one should pray for their forgiveness and mercy at all times. Among the best supplications is that which the Prophet, peace and blessings be upon him, used to say: "O Allah, forgive him, have mercy on him, grant him well-being, pardon him, honor his abode, and expand his entrance..."