Is likening the narrator to the Lord in His creation of events and characters in a story considered polytheism (shirk), and how can one repent from that?

Likening a novelist to the Lord is false and an act of disbelief (kufr), because it likens a creature to the Creator. Allah is the Creator, and everything else besides Him is created. Whoever likens a creature to the Creator has equated dust with the Lord of Lords. Allah, the Exalted, says: "Is He then who creates like one who does not create? Will you not then remember?" (Quran 16:17)

If the intention was not to equate, but rather it was a slip of the tongue and a poor expression, then she must seek forgiveness and repent.