What does 'her' refer to in Revelation 18 when it mentions the kings committing fornication?
In Revelation 18, 'her' refers to the world system, identified as Babylon, the great city. The kings of the earth committed fornication and lived deliciously with this world system; they bewail and lament her when judgment falls and her smoke rises.
Les explains that the kings of the earth who committed fornication and lived deliciously with "her" are dealing with the world system. In Revelation 18, they bewail her, lament for her, and see the smoke of her burning. They stand afar off for fear of her torment, saying, "Alas, alas, that great city Babylon, that mighty city! For in one hour is thy judgment come." The merchants of the earth also weep and mourn over her because no man buys their merchandise anymore. Les then asks what is going to happen, pointing to the coming judgment on this world system.