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.