At best, the police could say to Julie they are building a case against Bamber.
If he confesses & says he discussed the massacre with you, you could be charged with 'perverting the course of Justice'. If proven.
Doubtful that would be enough to scare Julie to compile such a comprehensive false WS & commit such serious perjury. Better to keep quiet. Espescially if Bamber really had not discussed it with her.