How should I explain to my non-catholic friends that priests should not disclose what they hear in confession to the police?
For a priest to disclose what he hears in confession would bring automatic excommunication. See: Seven Offenses Bringing Automatic Excommunication. The Church's Canon law has very precise regulations regarding the Seal of Confession.
I have never heard of priest going to the police with a crime heard in confession. Further, penitents always have the right to confess "anonymously" that is behind a screen so they cannot be identified to the priest.
Good luck with this - and God bless,