Fastred may leave for Riften as a result of a choice in the "The Book of Love" quest. Boti and Jofthor have a scene where they talk about Fastred as if she was in Ivarstead which will play even if she left for Riften at the end of the quest.
Boti: "You have to let Fastred follow her heart. You can't tell her what to do."
Jofthor: "She's our only child, Boti. Am I to throw her to the wolves?"
Boti: "You're hardly throwing her to the wolves. She just wants to see the world... just like I did when I was her age."
Jofthor: "And if she decides to leave Ivarstead, what then? No, no... I can't bear the thought."
Disallow if Fastred left Ivarstead at the end of "The Book of Love" quest.