Hi, thanks for your question. My name's XXXXX XXXXX I'm going to assist you with it.
There is no comprehensive legal definition of defamation but a statement will be defamatory if, for instance, it lowers the claimant in the estimation of right minded members of society, disparages his business or exposes him to ridicule or contempt. Where defamation is in writing it is libel and where it is spoken it is slander.
Truth is a defence to defamation. However, the defendant would have to prove that the information was true and this is not always easy, particularly if a statement is only partially or 'roughly' true. Likewise, an opinion is not generally defamatory but it must be fair, reasonable and have some basis to it.
To be frank, the real issue with actions for defamation is the expense. Claimant's need very deep pockets indeed in order to bring such actions. However, if the defendant is unlikely to be able to show that his statements are wholly truthful, you could consider instructing a solicitor to send a letter to the individual explaining the law relating to libel and slander (and the consequences thereof) in a bid to have him remove the statement. This shouldn't cost more than a couple of hundred pounds.
If the information is posted on an online forum or other third party webiste, you can also complain to them and they may remove it. They are likely to have terms and conditions dealing with defamatory comments and are likely to be cautious with regard to any potential joint liability for publishing it.
I hope that's helpful and just let me know if I can clarify anything for you.