Can a person speak loshon hara about someone who is not a shomer shabbos?
The prohibition of lashon hara applies to those who are considered “amitecha,” which Chazal interpret to mean “those who are among you in Torah and mitzvos.”
For those who leave Torah observance, the prohibition does not apply.
However, the reason for which the prohibition does not apply is to allow for the denigration of the wicked, to ensure that others are not influenced by them. One must not denigrate somebody without purpose.
In addition, many non-observant people are classified as tinokos shenishbe’u: They are non-observant because they do not know better. In this case it is not permitted to speak lashon hara against them, unless this is necessary for purposes of protecting others.
See Sefer Chafetz Chaim, chap. 8.