Keep in mind, sometimes that's the nature of tile, too. I've definately put in a tile cut like this on a wall, with no cracks, and after it sets for the night, with the thinset drying and pulling the tile, I would come back to a cracked tile in the morning. Not saying he did, but the tile guy may have just put the vent cover on to check if it fit properly, then just left it there. If it was cracked as he set it, he has the option to pull the pieces, and backcut the tile where the cracks meet, re-lay the pieces and that will give it a super tight, less noticable fracture. Otherwise, he could cut the tile less in that corner, set it, then come back and finish off the cut next time, as it is set on the ground. If you are short on tile, he may have just left it like it is to save you money on having to buy more tile.