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.