Based on the rules we have just studied, the rules of loshon hora vis-a-vis the totally non-observant Jew become clear.
(1) The average non-observant Jew today has been deprived of a meaningful Jewish education. Whether or not he has been exposed to Orthodox Jewish life, society has prevented him from taking Torah seriously. Rambam compares such a person to a tinok shenishbah, a kidnapped Jewish child whose kidnappers reared him with non-Torah ideals, and whose transgressions are committed out of ignorance. Such a person may be in the category of both “your people”, and “his fellow”. It is our obligation to educate him with love, care and sensitivity and it is strictly forbidden to speak loshon hora against him.
(2) A non-observant Jew who did receive a meaningful Jewish education, has been taught Torah values and knows that what he is doing is wrong, but claims that observance is too difficult, is a rebel due to temptation. He is considered part of “your people”, which means that loshon hora may be spoken of him only if it is for a constructive purpose.
However, he is not considered “your fellow”, and consequently the prohibition of onaas devarim, causing hurt through the spoken word, will not apply in his case. Social pressure may be used to encourage observance even if he will suffer embarrassment in the process, provided that all the conditions of constructive speech are met. Non-constructive speech that is either derogatory or harmful would constitute loshon hora.
(3) The classic apikores (heretic) who is knowledgeable but rebels is considered neither your brother nor a part of your people. If there exists such a person today, one may speak about him without any precondiltions.
Sefer Chofetz Chaim, 27 Cheshvan, pages 162 and 407