There is nothing as damaging to the cause of truth as being caught out in a lie. Make sure you have the facts at hand. If you don't have the facts at hand, do not comment - go find out the facts. You might say ``That's an interesting issue, let me research that and get back with you.'' That does two things: it makes you look impartial, and it gives you an excuse to get back with him preferably at a time when the FUD is not so recent in his memory.
Using facts is the best defense against FUD#2 (outright fabrication). However, be aware that the proper groundwork must be laid. In particular, you must come off as being a credible commenter, meaning that you must demonstrate a level head and present the appearance of impartiality.