This certainly makes it more likely she won't spread the information. However, I still think Kei would have preferred she didn't know, considering they are face to face at school all week long. In the end for Kei this is nothing but a job, especially now when Mayu already accepts him as a guy as well, without neither needing to run away. I'd even go as far as saying Kei should maintain a dose of shame over this job so that he remains more careful about it. He wouldn't want random guys from the school to learn about it, I imagine. Although he's scary enough (in his male form) not to get bullied easily, but still.