Hilchos Lashon Hara chapter 10, section 6

Sometimes you may tell others even if the person spoken about does not yet know. This is allowed when you see that it will help, as long as all the conditions above are met (see Be'er Mayim Chayim).

For example, you see that the speaker is the kind of person who will do it again. Just as he put down the victim to you, he will go and put him down in front of others too.

We already wrote that you must first correct him. So this only applies if you corrected him and it did not help.

Sadly, almost everyone believes lashon hara. Once the speaker talks to other people, it will be very hard to convince them the story was false, because first impressions are the strongest.

So you may go to the people he is likely to talk to, and warn them that he may come and speak falsely about the victim. Then they will not believe him later. Instead, they will correct him. And when he sees that his behavior brings him shame, he will stop.

All this is certainly allowed, because it helps both the victim and the speaker. You also keep the mitzvah of correcting others.