Hello Antoninus!
Same simple question for you:
If a priest rapes a child and confesses it to another priest during confession, would that priest to whom it was confessed notify the police about said child rape?
YES or NO
He can make confession to the police a condition of valid absolution, however, meaning that if the perp wants to keep it a secret, he's accomplishing nothing by going to confession.
No. He also wouldn't pat the guy on the shoulder and say, "There, there, I know you're really sorry, go in peace". He would rip the penitent a new one and tell him he had to go to the police himself. Remember, the penitent is at Confession in the first place because he knows he's done wrong and he wants to make it right with God. The priest is going to tell him he also has to make it right secularly.
You're not actually absolved until you complete your penance - the one time I couldn't do mine immediately (I went to Confession at lunchtime and had to go back to work) the priest told me to do it before I went to bed. Believe me, it was like a sack of cement across my shoulders all afternoon.