There is little that can be done at this point other than a do over. You may be able to waterproof the sides and bottom and perhaps use a product like Kerdi Fix to seam the waterproofing to the tile itself in the back but this will not address the front edge.
While you stress over this ask yourself this question. "Who is the client in this arrangement?"
How did you find this tile crew?
Ask them to give you in writting a warranty for any damages caused by water damage. Or ask them to reinstall the backer board as per manufactures guidelines.