Is there the option of giving anonymous notice of this, Bodi. Maybe the police can't act on that, but, if they got more things like that, on the same man, or even with a single anonymous letter about him, they may be taking other signals way more serious.
If that person you know doesn't want you to officially mention it to the police, then you doing that, and mentioning the name of the victim would be another breach of trust for the victim, and, it could make it harder and harder for her to trust anyone.
Yet, the man who did this, still is in a position where he has easy access to people vulnerable to abuse. And, he'll probably have a nose for that too. So, not mentioning it, hardly is an option, now you know about it.
Anonymous letter, without mentioning the name, or anything that can identify the victim, may be the only reasonable option.
And, it may be, that, if said man gets scrutinised, and exposed, that the one confiding this to you, can find courage to file a complaint, and to find help in dealing with it.