Shemiras Halashon Halachos

Hilchos Rechilus, chapter 7, section 3

It makes no difference if one tells Reuven that someone acted against him or tells Reuven's wife or relatives, for they will certainly regard it a wrongdoing and bear a grudge. Therefore it doesn't help if one warns them not to repeat the information to Reuven.

Hilchos Rechilus, chapter 7, section 4

It makes no difference if one says the rechilus to a Jew or non-Jew. In fact, the latter is even worse as it can lead to great harm and misery (besides other reasons explained above in part 1 paragraph 11). Some people are very guilty of this. They tell a non-Jew that the work or product a Jew sold or did for them was faulty and this commonly leads to harm and misery, and can even cost the Jew his livelihood.