From the strict letter of the law, may one talk Lashon Harah about a Mechalel Shabbat?
It depends on why the person is a mechallel Shabbos (not all mechalelei Shabbos were created equal…). If the person was raised in a non-religious society, according to numerous poskim he is considered a “tinok shenishba” a child that was kidnapped and raised as a gentile. Such a person although he doesn’t keep the mitzvos, is not considered as purposely sinning, because his education misleading. Therefore, we may not speak lashon hora on such people. However, a person who was given a Jewish education, but decided to become irreligious, on such a person there is not technical prohibition to speak lashon hora. As a side point the chofetz Chaim states that even on a rasha, it is not a good idea to get into the habit of speaking lashon hora about them, because we will get accustomed to speaking lashon hora. Another point worthy of mention is that it is permitted to speak lashon hora about people that are openly apposed to torah and try distancing us from doing mitzvos etc. This is because it will cause us to distance ourselves from these evil people.
Chazon Ish Y:D 1-7, O:CH 87-14, Derech Sicha 2 pg. 456-7, Chut Shani ( Shabbos) 2 chap. 40-3. Shulchan Shlomo (Yom Tov) 512 ftnt.1,4, Shevet Halevi 2-172, Igros Moshe E:H 2-20(4), Ohr L’tzion 3-chap. 19-3, Oz Nidberu 14-69, Daas Yehuda pg. 246, Teshuvos V’hanhagos 1-478.