Unless I am mistaken…when the grout dries on he tile, the layer made a huge mistake. I have seen this before and was told by the manager that it is virtually impossible to take off. I would ask the customer if he could get the tile layer to remove the grout left behind first…
Call Big Bad John…and take his advice. You run a great risk of ruining the new tile finish/glaze by mechanically removing the grout.
Wish you the best…just my thoughts. I have walked away from a couple of bath jobs just like this due to the nature of the beast. The risk/reward ratio was not in my favor. If not, I walk…or run.
Dave