Yes as Patricia alluded to, cement board alone is not waterproof.

Thanks! I will mention that. He’s using the Schluter System. https://www.schluter.com/schluter-us/en_US/Shower-System/c/SS

Patricia, why would you suggest NOT doing a border tile on the shower floor with the mosiac in the center?

Make sure he waterproofs the shower or else it will be a nightmare! The staggered look will go well with your space.

@Patricia - When you do 33% stagger, does it matter if you do every other layer the same, or if it looks more like a stairstep?

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

Do a brick joint with your 12x24 wall tile. The length is only double the width. In that size the warpage should be very minimal and you should easily achieve a brick joint pattern. And yes, the border on the floor is an excellent design transition for your wall to floor as it eliminates the abrupt change of pattern. And most importantly, see Creative's link above.

Wonderful info on choosing an installer. Thanks!