What responsibility comes with truth?

Let's say a friend is taking a herbal supplement for depression, like St. John's Wort. He tells you how much better he feels after taking it for a few weeks. Later, you read in the newspaper about a long-term, comprehensive study that says it proves that the herbal supplement has no medical affect. Do you tell your friend about the study?

And what conditions, if any, would make you change your answer?