Is it permissible to speak lashon hara about a Jewish (non-religious) person that knows they are doing something wrong and still do it countless times? What is the source for this in Chafetz Chaim? I've heard that it's allowed, and I've heard that it's not.
From reading Sefer Chafetz Chaim, it appears that the answer to this question is usually no (Chafetz Chaim 4:4, 8:1, 8:4, 8:13). R. Binymin Zilber in Shut Az Nidberu 14:60,69 understands that speaking evil about non-religious Jews is permitted (see below), but discourages it because we shouldn't be increasing negative feelings between ourselves and irreligious Jews.
Exceptions to this rule are sinners who are heretics and sinners whom one is allowed to hate (see Klal 8 for details), and perhaps about people who are engaged in a fight. However, in the case of a heretic, one can only speak lashon hara if
Similarly, in the case of speaking evil about two people fighting, lashon hara may only be said if three conditions are met: (1) the speaker was witness himself to what he's going to say (2) his intent is to stop the fighting, not to degrade the subject, and (3) he feels that there's no other way to end the fight. (See Beer Mayim Chayim 28, though, that lashon hara in such a case might still be prohibited to be believed)
A final exception exists where speaking about a sinner will help others stop him from engaging in this sin, or otherwise help him do teshuva (see C.C. 4:5-6 and 10:4). However, the Chafetz Chaim lists 7 conditions (in 10:2), but each of them come with several caveats/complications, so such an issue is probably best determined on a case by case basis (i.e. CYLOR)