Hilchos Rechilus, chapter 9, section 4

Even though this is a great mitzvah and part of bringing peace, he must still think very carefully and not rush to tell such a thing.

He should tell only if he knows that afterward the person will be careful not to go to that place alone, so the other man will not hurt him, or will find another way to avoid harm.

But if he knows that telling will make the person angrier, and he will go and start a fight with the other man first, and this will lead to an even bigger quarrel — in such a case it is forbidden to tell him. So from the start he must think carefully about how to tell it in a way that will end the quarrel and not make it bigger.