Tile grout crack with newly installed tile over backer board.

[Catherine Ann]

I recently contracted with a home repair company to install a tile floor.  He came highly recommended by a friend.  The installer suggested using two layers of 1/4" backer board instead of one 1/2".  Now the grout is cracking between the tiles in the heavily traffic areas.  All tiles were installed two months ago.  The Installer says he'll fix by removing the grout and screwing additional screws  through the open grout spaces to tighten the backer board slabs to the plywood underlayment then regrout. Does this sound like a fairy tale fix?