Hilchos Rechilus, chapter 9, section 3

Now we will explain another case where there is no ban of rechilus. For example, he heard someone say: "I will hurt so-and-so in such-and-such a place — I will hit him, or curse him and insult him." Or he heard from him that he wants to cause that person a money loss.

It depends. If this man is known to have done such things to other people several times, or he sees from the situation that it is not an exaggeration and he really means to do it, he must tell the person it is about. Perhaps then he will know to be careful and avoid the shame or the harm. But he must also be careful that none of the details we listed above are missing.