Natsume is at home early one evening when he hears a knock on the door.  He sees Tohko outside, asking to be let in.  But when he opens the door, there's nobody there, and a gust of wind rushes past.  Another Youkai is after the book of friends!
What I should've done then
En ese momento debí 
despedirme de ella por él.
En aquel momento debí haberme despedido de ella en su lugar.
Naquela hora, eu deveria ter me despedido por ele.