Would he have been able to write off taxes against a smaller car?

Nope. Does he need a vehicle that large? Probably not. Does it come in handy when hauling our displays to conventions, etc? Yes.

The law does not specify what is and isn't legitimate business use. He's a business owner and is using the vehicle for business purposes. IMHO, that's just taking advantage of the incentives to business owners offered by the government, not bilking the public.