No arguments on the dick part, but if the vendor-supplied software has basically an "overclock" tickbox built right in (some videocard driver packages have this), he may not be 100% wrong to take them back under warranty. Under US law at least, there are implied warranties for "fitness for a particular purpose", which a company cannot always disclaim once that company has implied them (this varies by state to some extent). A good example is a pickup truck that comes with a ball hitch and is shown in the TV ads towing a camper. This creates an implied warranty that the truck is fit for towing something equivalent to the camper shown in the ad, and the vendor placing scary language otherwise in the warranty/manual may not necessarily dissolve that warranty. Likewise, nVidia might be worried that placing a user-accessible 'overclock button' right in the UI would create such a warranty and make them liable for implied warranty claims from OC'ed laptops.
(Whether OC'ing the GPU should be able to permanently damage the GPU or laptop in the first place is another issue, but being covered by some other threads in this discussion.)