What should you do?

Suppose you find out that someone you know did something horrific to someone else. The victim (understandably) doesn't want to go public. What should you do?

It's surprisingly hard for me to answer this question. Morally, the perpetrator should face consequences for their actions, but there's no obvious way to make that happen without also causing consequences for the victim. (I guess you could question the premise that the victim should be allowed to choose whether to go public, but I think the common agreement is that it's their right to choose).

To some limited extent, you can can tell individual other people about the situation, but (depending on context) there's a chance that this still risks outing the victim, and regardless you could only plausibly reach a tiny fraction of the people who need to know.

You can maybe hint vaguely about a person's untrustworthiness in public, but without a concrete reason this is unlikely to be persuasive in general.

I see a lot of stories in the news about people who have done terrible things finally being outed, and it's clear that some non-trivial set of people knew about the behaviour for a long time previously, and then the broader public (understandably) feels extremely upset that these people knew and yet did nothing.

To be explicitly clear, some of the people-who-knew also didn't want to do anything about it, even if they could. Which is its own kind of awful.

But as best I can tell, in many contexts, even if you badly want to do something about it, you can only really do anything once at least one victim is willing to go public, and until then.... you can only look on in horror? (Please tell me I'm missing something, I would badly love to be missing something).