So the mortar adheres the tile to the concrete board or does the mortar adhere the tile to the wood?
I have removed tile from both wood (plywood subfloor) and cement board (underlayment in the bathroom). In both cases the thin set was so well adhered that either chunks were left behind or I wound up tearing up the underlayment pretty bad and ended up removing the substrate and laying down a fresh piece.
You might get lucky and the poorly mixed mortar is easy to knock off the substrate. Otherwise, if the tile is adhered to the cement board, I would be inclined to cut out a section of the cement board with the tiles and place fresh piece.
When I tried removing my tile, I did try a oscillating tool and a carbide abrasive to knock down the lumps . . . I wouldn't recommend that. Perhaps and angle grinder with a diamond surface grinding wheel would be more effecting. I also used an SDS+ hammer drill in hammer only mode with a chisel blade to knock the tiles looks.
Of course all of this presumes that there is not some sort of waterproofing that will be interrupted.
I have to ask the question. You have a "usual tile guy", presumably some sort of professional you have an established relationship with, who made a mistake/error, so why are you the one figuring out how to fix it?
