For instance, if a penitent confesses murders, or obsessive murdeous fantasies (with acts, e.g. getting ammo), or pedophilia, or the like?
Can the confessor say, "I can't grant you absolution until you turn yourself in to the police"?
If the confessor can't go to the police himself, is there some other way he can protect the innocent? (Can he tell Mrs. Parishioner Jones and say, "No, you shouldn't let little Jimmy go on a fishing trip with Mr. N"? Is there some way he can drop the dime on the guy without sacramental violation?)
Ears perked.
But how does the priest know it’s Mr. N in the confessional? Unless he really and truly KNOWS, rather than just suspects, even if he was willing to violate the Seal, he’d be slandering Mr. N by telling Mrs. Parishioner Jones not to let him take her boy fishing.
There's a classic old Alfred Hitchcock movie about this theme, called I Confess.
The priest can make going to the police a condition of absolution. However, he cannot do or say anything that would tie a particular sin (or anything else said in confession) to a particular penitent.
(It would seem to me that in some circumstances he could attempt to deflect suspicion from someone he knew to be innocent, since that's not necessarily tying a sin to a particular penitent. He just couldn't say anything more about how he had acquired that information.)
As has been pointed out, in a traditional confessional setting, the priest often does not know the identity of the penitent anyway, since he or she is behind a screen.