No, I wouldn’t think it would be a sin. There could be a million reasons why he bought those books – curiosity, working on ecumenical project, looking to switch, wanting to be more secure in his own faith, etc.

As outside observers there is no way we can know what his intentions are, but God does and that’s why it’s only God who can make judgments.

Personally, I would take no action – it’s none of my business. On the other hand, perhaps I would strike up a conversation with him to learn his perspective.

They were indeed on the Index of Prohibited Books. While the Index does not exist, we still have the responsibility to avoid books that are not edifying or are harmful to our Faith. The scenerio you gave did not have enough information to make the determination, and as laymen we could suggest or encourage the man to seek the advise and guidance of his priest if he made statements that would lead one to believe that he held these texts to be of equal value, or inspiration as the Bible. One can and should also defend the Faith to the extent one is able.

The Index was established at a time when those who could read were being assaulted by new and strange ideas that were gaining popularity along with a new invention (movable type) Since Vatican II the emphasis from many of the clergy in the Catholic Church has been to encourage learning among the laity, and to treat people as Adult Christians. Prior to Vatican II, (in the world in general, not just the Catholic world) most people did not have a High School education and were more likely to not have the capacity to understand some of the concepts that those who attacked the Church wrote, or to know how to reply to those attacks. While many of the false allegations against the Catholic Church and dangerous ideas are still in circulation, we do today have access to resources like the internet where we can find responses, (not unlike in the past) if we know where to look.

Even in the days of the Index there were legitimate reasons why a Catholic would have copies of the books you mentioned, and read them. Usually if someone be they a priest or layman was a student of religions, or Social Sciences, History or had a resonable excuse to read these books, they could seek permission to be exempted to read the books, as long as it was within the scope of their studies, or research for writing a book.

So while the Index no longer exists in a published form we do have the obligation to avoid those publications that would pose a threat to our Faith or Morals. Since the Index was never a dogma, but a use of the right of the Church to govern the morals of Her subjects, and instruct as well as protect the Faithful.