Hi There,
I have work on a lot of reputation management for my clients and negative reviews is an on-going battle. There are a few things you can do to improve things for your client.
The quickest and easiest way is to published fresh, relevant and current news about the brand. If you are distributing press releases or news stories through the right channels you will get great coverage. If possible ty to avoid the free publishing sites as many of them are are associated with other negative attributes. If you have a budget to work with I would recommend PRWeb, Marketwire or 24/7PressRelease wire.
I have successfully pushed negative reviews off page 1 in Google by using well writen, relevant and news-worthy press releases.
I have also had some good results with Google+, by linking google plus into the brands exposure Google will give a high ranking position for the Google+ brand page and even display images. I have found this diverts people away from negative reviews.
Has your client responded to the negative review? Offered a refund, apologised etc etc. Often a reviewer will remove the complaint etc if handled in the right way
Also, depending on your location (and local laws) some review sites will amend the title tag that shows up in a Google result to make is less 'nasty' to avoid a legal battle with the brand.
There are a lot of things that can assist with negative reviews - hope you find some of the above helpful