Kento Nara had been thinking that Manatsu would surely go mad in later chapters according to the plot's development.
He deduced this logically. As Miss Gentera did not arrange for a psychiatrist to provide counseling for Manatsu, it would be very illogical if he did not go insane in later chapters. The whole book would be ruined if that was the case.
Even if the storyline had later incongruences, the novel was still considered to be of good quality. However, Kento could hardly recommend a book that had lost its good public reputation to others.
Nonetheless... judging from the words '50 years later', Kento Nara knew that this was Miss Gentera's trump card.
Time could change a lot of things, and the easiest things to be changed were thoughts and personality traits.
Maturity of thought corresponded to the progression in the human lifespan - children would think immaturely, while adults would be more reasonable.
