Doesn't make sense. Equine dentists do so much more than just round off sharp edges; they look for any bleeds or abscesses caused by the sharp teeth, look for teeth that need to be pulled or fixed, and make sure that the horse's teeth can grind properly.
How would corn fix a horse whose molars are worn down in odd places so they can't grind properly? How can corn reduce sharp edges on surfaces that aren't being ground? Unless you're feeding corn kernels, there is no way that it's even strong enough to grind down any teeth at all; and I'm fairly sure it's a bad idea to feed corn kernels in the first place; that's a good way to crack a tooth. My advice? Call up this dentist and have him talk to your dad.