Praying for the dead is not a biblical concept. Our prayers have no bearing on someone once they have died. At that point their eternal future is confirmed. Either they are saved through faith in Christ and are in God’s presence where they are experiencing rest and the joy of being in His presence, or they are in hell or torments. Luke 16 and the story of the rich man and Lazarus the beggar is a good illustration of this.
Paul was confident that to be absent from the body was to be at home and present with the Lord (see 2 Cor. 5:1-8). He also said, “For to me, to live is Christ and to die is gain. Now if I am to go on living in the body, this will mean productive work for me; yet I don’t know what I prefer: I feel torn between the two, because I have a desire to depart and be with Christ, which is better by far” (Phil. 1:21-23).
The only possible basis someone might use (though wrongly) for praying that the dead might find rest is Revelation 6:10 where martyred saints seem restless and cry out to God to avenge their blood. But in the very next chapter, the martyred saints in heaven are seen in total rest (Rev. 7:9-17).