goodroi - 5:45 pm on Jun 28, 2012 (gmt 0)
If it is a legitimate comparison and they are not claiming to represent your client's brand, it would likely be legal in the USA under the Fair Use principle.
I would suggest using some simple, old fashion reputation management methods. Launch multiple sites (repair site, forum site, blog site, etc.) to crowd out the top ten. Also think about building up the social profiles (facebook, twitter, etc.) and then pointing links to them so they also rank in the top ten.
If what he is doing is legal (check with your lawyer) then you can also return the favor and create a comparison page to start ranking for his brand names.