When Travis pulled Andi in the pool, she hit her head and died. The rest of the book is her fantastical dying dream. The book prior to this had hints of strangeness, but after this is when the strangeness really started.
Riley mentions a previous girlfriend briefly, so we can assume that she was dumped by Riley in a similar fashion to Andi. Scarlett then developed an undying obsession with elder gods as a result of being so unjustly scorned. After the obligatory sobbing and sprawling on top of driveways, she stumbles upon Uncle Neil's shop and steals one of his Eldritch beverages.
She drinks out of it and becomes a demi-elder-godess; granting her eternal life and impossible beauty. With these new found powers and smexyness, she starts a cult with the intention of eventually becoming powerful enough to take control of Cthulhu. Being a demi-elder-goddess also explains why she doesn't perish when she see's Riley's true form.
Note: Scarlett does not recognize Riley at the beginning of the novel since he uses a different human disguise every decade to avoid recognition.
- Scarlett has blonde hair, but that doesn't disprove the theory.
- Alternatively, the events more or less do happen in a similar way as described (although Andi probably completely missinterprets Cthulhu's motivations). Andi really is mad, but this madness is exactly what she herself perceives as love.