It is forbidden to relate that someone has been remiss in matters of Jewish observance — be it a transgression prohibited by the Torah, a rabbinical prohibition, or even a breach of custom. Such statements are derogatory by the Torah’s standards, and are thus forbidden.
Therefore, it is forbidden to mention an incident in which one of the people involved transgressed a halachah, even in a society where that particular halachah is commonly ignored.
Sefer Chofetz Chaim, 2 Tishrei, page 52