Shemiras Halashon Halachos
Hilchos Rechilus, chapter 3, section 2 It is rechilus to say to anyone that Reuven spoke against Shimon, because the story will
spread and will cause enmity between Reuven and Shimon. Even if one warns the person one
speaks to not to spread the story, and even if one knows for sure that he will not spread the story,
it is still forbidden, because merely saying that Reuven spoke against Shimon is itself generally
Hilchos Rechilus, chapter 3, section 3 4 How much more is it forbidden to say that someone spoke against his sons or relatives, for
the sons and relatives will naturally be extremely upset if they hear of it.
If one tells someone that Reuven spoke against Shimon so that the person one tells this to
should rebuke Reuven for what he did, see laws of lashon hora paragraph 114.