Questions about shower wall tile install
Here is a post I found on JLC's website.

Re: Vitreous or Semi-Vitrious tile in shower? 

The absorption problem is behind the tile - not on the surface, so sealing the face of the finished job will not prevent moisture from penetrating the tile body (via moisture seeping in through grout joints. 
 Unless the shower will be used so much there is not enough time for the installation to COMPLETELY dry out in between uses, semi-vitreous tiles are a poor choice. Whether or not this (degree of use) is the case, the density of the tile grout, and the adhesive contact are critical: grout mixed, applied, packed, and cleaned properly with an absolute minimum of cleaning water; 95% minimum adhesive contact between backs of tiles and setting bed. If you remove a tile and can see ANY trowel marks - there is not enough adhesive contact: voids in the adhesive layer will collect moisture and guide it into the tile body. When both are saturated, mold will grow: once established, you cannot kill it.

Make sure that the customer knows that impervious tiles are the best choice in wet areas. Semi-vitreous tiles are not the worst choice, but I would not recommend them. A disclaimer in your contract will help once discoloration or mold appear. 

End of post

It sounds like a really bad idea....