Use of a Book by One Who “Learned and Then Abandoned Observance”

Shalom uverachah. An author who wrote a halachic compilation or Torah book and received approbations for it from leading poskim at the time of writing, and afterward cast off the yoke of Torah and mitzvot — is it permitted to study from his books that were written when he was observant of Torah and mitzvot? And likewise, what is the law regarding a scribe of sifrei Torah, tefillin, and mezuzot who wrote while he was observant of Torah and mitzvot and afterward cast off the yoke?

Shalom uverachah 

 There is no prohibition against using what he wrote when he was righteous and presumed to be God-fearing. 

The ruling of our teacher Rabbi Amram Fried shlita. 

Based on Shulchan Aruch, Yoreh De'ah, siman 1; there, siman 281; and Orach Chaim, siman 39. And Ketzot HaChoshen, siman 34, se'if katan 23. 

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