Hilchos Rechilus, chapter 7, section 4

It makes no difference whether he tells rechilus about a Jew to a Jew, or about a Jew to a non-Jew. In fact, telling rechilus to a non-Jew is an even worse sin, because the story might cause the Jew harm or pain.

Some people stumble in this, and tell a non-Jew that something he bought from a Jew is faulty.