My father in law talks loshan hora. As a matter of fact most of his conversation is name dropping and then talking in depth about them. He is an atheist. I was thinking about mailing him the “Guard Your Tongue.” And anonymously. Is this forbidden?
Technically you are allowed to send him the “Guard Your Tongue”, because he is you father in la and not your father, and this aspect of Kibbud Av V’em will not apply. However on a practical level, it is hard for me to understand your situation. You write that he is an atheist, therefore it is quite doubtful that he will be interested in learning, and keeping the halachos of lashon hora. But on the other hand he doesn’t say names when speaking lashon hora, which sounds like he does have interest in keeping the halacha. Therefore I need more of a clear picture of the situation in order to clearly answer you.
Shulchan Aruch Y:D 240-24.