and he is going to use a sanded grout on the wall. Will that not leak after a while?
You have another thread going about this similar thing, but you brought up another thing here that needs to be addressed and that is about grout providing a part of the waterproofing of the tub/shower walls.
Sanded grout is used for joints of 1/8" or more, wall, floor, ceiling, inside, outside, whatever. Grout doesn't care where it's going, it only cares about the size of the joint.
Grout does not perform any function of waterproofing. If your shower or tub surround cannot contain every bit of water prior to the very first tile being installed, it will not contain the water once the tile is installed either.
Construction of the shower walls includes something to provide for the waterproofing, either a moisture barrier of 4-6 mil plastic sheeting or 15 lb roofing felt installed to the framing and overlapping the pan/tub flange or shower pan liner then the cement board-or- a surface applied waterproofing membrane is installed on the cement board, such as a liquid like Hydroban or Redgard or a sheetstyle like NoblesealTS or Kerdi. If none of these options were done, your installation shouldn't be proceeding anyway.
If your installer has built a shower pan for you using a shower pan liner and mud, and you are not using a preformed shower pan or just doing a tub surround, then you also need to be certain these things were done correctly in constructing the shower pan for you. If you are not certain, but you took pictures or could describe the steps, post the pictures od describe the events.
--The shower pan liner cannot be installed flat in the floor. The installer must first install a sloped mud bed so the liner is pitched. There will be 2 mud layers with the liner sandwiched between.
--The shower curb must not be formed by nailing cement board to it.
--No nails in the bottom 8" of walls. If you saw nails/screws in the bottom of the walls before the top mud base was installed or the walls went in after the mud base went in and you see nails/screws anywhere within 5-6" of the bottom of the walls...
--No pressure treated lumber. If you saw any wet, greenish colored lumber used, it will dry out, twist, warp and break apart your tile.
--Knee walls (half walls) and site built niches will require some form of surface waterproofing.
--Benches.. It's amazing to me that some contractors think they can take shower liner, run it up and over a bench, nail through it, and think it's waterproof still.