What is even the point anymore about keeping it a secret. Sasaki has already expressed her desire to come clean to the cops! All keeping it a secret does is protect his own ego. Destroying the evidence won't even save Sasaki, the guilt that he is destroying everything around him to protect her would destroy her. With no evidence she will be guiltridden or be declared a madwoman for spouting nonsense without any evidence.
Plus, the information that she was raped by the teacher is already known to the media, so she isn't even protected from being a known rape victim.