Someone on a frum online writers forum posted an enthusiastic recommendation for a book. The author is a frum yeshiva man. I happened to have read part of the book and I know it contains some blatant immodesty. Am I allowed to post a warning on the site telling people about this or is this loshon hara?
Yes, you may post this warning. This is considered a bona fide “toeles” need and purpose to share this information to save people from harm, in this case spiritual harm. If possible you should first notify the author of his error and that you plan to take the above action. Perhaps he will choose to remove his review first.