As I remember, you can give the red notebook back even if you steal the key. I think there are two options here - if you give it back before cutting her hair, she just calls Kat "weirdo". If you cut her hair first - I think she calls Kat b**ch.
I was trying to speak more vaguely to avoid spoilers.
I meant the very end, when you find her dead body. I picked the lock to the theatre room using my hairpin, which I found at another desk, so I still had her notebook when I found her body. It was her hair that I cut for the ritual, so I felt kinda responsible for her death.