Guys, you're forgetting that Sayu is a runaway, and is still on the run, staying at a stranger's apartment. The second she calls the police for the attempted rape, her whole situation is fucked. She let him visit MC's place because rapist boi was threatening to tell the manager about her past, which would lead to an investigation by the police, which would lead to her being sent back to Hokkaido. Same thing would happen if she reports the attempt rape. As much as I hate him and want him in jail, that won't be happening until Sayu is ready to face her past and go back home, and that's if the author remembers about rapist boi.