In the UK you could contact trading standards as he's technically "passing off" as your client. Not sure about the situation outside of the UK though.
Basically you need to ringfence your clients brand term - is it unique or is it a generic keyword phrase? Unique terms are much easier to protect.
I assume your client's site is number 1 for the brand search? If so, that leaves the next top 30 to ringfence. Basically you need 30 other sites to optimise for the term (or at least, pages on other sites), but you need to do in a way that they will stay there.
In the short term, try press release distribution via PRweb, etc - those releases tend to rank well for brand terms (make sure the brand name is in the title of the PR
) for a few weeks at least.
Also, contact forum owners to ensure them that your client isnt responsible and ask them to removing offending threads - that will prevent them from being indexed in the first place. Keep at them if they don't respond - even register on the forums and respond to the thread to ensure visitors that it isnt your client.
These kind of thing happen - I've been working on several brand protect techniques for a while now to ringfence some of my own brands. There are a lot of @ssholes on the net!