Hilchos Rechilus, chapter 9, section 3

Another case where rechilus does not apply is if one heard Reuven say: If I meet Shimon
in so and so place I will hit him or insult him, or if one heard that Reuven wants to cause him
monetary harm. If Reuven is known to have done to do such things many times to other people,
or if one sees according to circumstance that Reuven is not exaggerating and will certainly carry
out his threat, one must reveal the matter to Shimon so that he can perhaps avoid the damage or
embarrassment. But must be careful that the above mentioned conditions are fulfilled.