Legal Question in Business Law in California

Yes, but be prepared for him to hire an attorney who will bring a SLAPP motion. I do suggest you consult with an attorney first, and get some good legal advice as to the litigation risks before setting down that path.

If the review is false, then you may have a cause of action against the author.

You really need to decide whether the cost of the litigation outweighs the potential benefits. This is true even if you intend to file your case in small claims court. It is not always easy to prove damages in this type of case.

I believe that even if there is some sort of defamation that proving that it was your firend may be difficult since it is posted on the internet.... I agree with my two colleagues above the risks may not out weigh the advantages

Sure, if you can prove not just 'his poor opinion of you' but all the elements of actual slander along with provable damages, and if there is a reasonable likelihood that you could collect anything if you actually are able to get a judgement for enough to justify the fees and cost of litigation. Anybody can sue anybody for anything. Winning and collecting are entirely different questions.

You may have a valid cause of action for trade libel if the review contained false statements of fact (as opposed to opinions). Before bringing a claim, you should consider whether the review has caused any actual damages to your business. I would be happy to discuss with you further.

As seen from the foregoing, a legal proceeding may not be cost/time/effort effective. It seems that you have self help options available. First, since it is your website, you should be able to remove the post. If not, it is often effective to write a response pointing out the fact that he did not even use your services and is not in a position to comment upon them!