Respond to the accusation publicly, making it clear that you are willing to work out a solution. Make sure that you have other reviews that are positive so that you are not stuck with 100% negative reviews. If the review violates terms of use then you can probably get it removed but it may take time. The other two steps can be done immediately. If you are behaving like the good guy and the other party is not then this becomes fairly clear just by what's posted online.

It's hard not to take things personally though, after working hard to do right by a client in the first place.