there is a water proof mastic for just this purpose that is water clean up and easily trowelable and would be cheaper and more effecient.
But, if he is just dead set to prove a point, I don't see why liquid nails would not work. Its just like Pulling a tooth because it has a tooth ache.
Liquid nails is not easily trowelable. How is he going to make sure that the surface is rippled just right for the adhesive to set. Wonder if the liq nails comes up between the tiles and how is this going to be cleaned up?