You Are a Prosecuting Attorney with a Strong Relationship to the Police

You are a prosecuting attorney with a strong relationship to the police force you partner with. An officer comes to you with a dilemma: his partner is about to testify about a robbery he was the arresting officer on, but he knows the partner's memory is shaky because he was going through a divorce at the time. The officer swears you to secrecy. How will you prevent the officer's partner from perjuring himself without breaking the trust of his partner?