A rebel due to temptation is someone who regularly submits to temptation in violating a specific mitzvah. While the prohibition against gossipmongering does apply with regard to speaking about him (as he is not a heretic), his situation differs drastically from one who occasionally transgresses a given mitzvah but basically is committed to its observance.
As we have learned, the primary prohibition against speaking loshon hora is “Do not go as a gossipmonger among your people”. One who embarrasses his fellow Jew transgresses the prohibition “A man shall not aggrieve his fellow”.
Our sages interpret the term “your people”, in the first prohibition as excluding only a rebel whose intent is to spite, one whose lack of regard for a given prohibition causes him to regularly transgress it. However, the term “his fellow”, in the second prohibition, excludes even the rebel due to temptation. Thus, it is forbidden to speak loshon hora about a rebel due to temptation but it is permissible to exert social pressure on him in order to induce him to mend his sinful ways.
It is permissible to publicize that a Jew regularly violates a particular halachah if it is clear that he is aware of the halachah and has chosen to ignore it, and it stands to reason that publicizing his behavior will induce him to change. This applies even if he transgresses the law out of temptation. However, one must be certain that all the conditions of toeles, constructive speech, including purity of intent and lack of an alternative, apply.
Sefer Chofetz Chaim, 25 Cheshvan, pages 158 and 407