Shemiras Halashon Halachos

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.

Hilchos Rechilus, chapter 9, section 4

Even though saving Shimon is a great mitzvah and leads to peace, one must be very careful to only speak to him after considering whether Shimon will avoid going to the place alone or do something else to save himself from quarrel. For it often happens that Shimon will become angry at Reuven because of one's story and deliberately go to fight Reuven leading to even greater controversy.