Its hitting his entire forum -- he may not be checking access logs to see if its hitting the main site... If the forums are, for example, contained within a subdomain of the main domain, it could easily be crawled independently from the rest of the site.

Speaking of access logs -- thats another big clue.. if you can check the access logs to see what ther referring IP is -- if they're uniques every time, or if all requests are coming from one spot -- this may give more clue as to whats going on...

Edit: Referral lines will also give a clue within access logs -- it should be documented within the logs what the linking originating page was.. if its google results, you should be able to identify that quickly.

-Corruption