Thats why forums exist...for discussion. Anyway, if what it was commented on that forum is no true at all you might want to suggest to your client to signup on that forum and comment back with proofs. Than he might politely want to ask the forum owner to disable the link since what the OP or whoever commented his services/company had no proofs and is speculating.
The idea might sound crazy, but if the guy who commented your clients company is speculating i believe he was not even going to ask to do that, instead he would have already signed up and commented back to prove that the op is speculating.
Other than that, just pray the forum owner go without funds and get his account suspended, or his DB corrupted in some way
