Tub only or is there a shower as well? Cracking grout is due to movement typically and since you state the tile is not flat it is probably rocking and the cracking will only get worse. No you cannot grout over grout. How was the shower constructed? Do you have progress pics? What is the substrate under the tile? The waterproofing system used?

Thanks for the help. Both tub and shower. Durock cement board, followed by Red guard. The tiles were set for a day or so, then he grouted it. It is not finished yet and no one entered there for a couple of days. So whatever movement is very minimal. 

Progress pics attached. That's what I thought, you can grout over old grout. His answer was "we have to put a second coat on it." What can I do from here?

While there are outliers, in general unsanded grout should be used in joints less than 1/8" wide. If your joints are wider, that's a concern. Based on the visible voids and pinholes, it looks like they might have not packed the joints fully, or used too much water in the mix or cleanup. Insufficiently mixing the grout, or not allowing it to slake after mixing, can also cause negative results.

Grouting over existing grout, it can be done but it's best done within a couple of days, and only because the joints were raked out too deeply during the cleanup. Grouting ful joints where the grout is cracked might not provide the results you are looking for. The second coat will only be as good as what it's being installed over. And it it's a only thin veneer or schmear, meh. You may end up with a blotchy installation due to one coat here and a second coat over there.

It'd have been better worksmanship for them to have caulked and not grouted the seam between the tile and the tub, as well as the other changes-in-plane in the shower; wall-to-wall corners for example.

With the cracked tile being close to a corner, there is a chance that there was too much mud buildup in the corners when the backerboard seams were mudded and taped. When the tile was placed there, it might not have been fully bedded in thinset. If the tile is bridging a hollow behind it, it might have cracked (or a stress riser created) when they tapped it down to bed it, or it might have cracked (or the stress riser formed into a visible crack) due to differential bond stresses on each side of the hollow as the thinset cured.

Sometimes we've had people pose photos of linear cracks through the tile field, with the cracks located over the seams in the tile backerboard. That's typically an indication that they did not mud and tape that joints in the backerboard. Differential movement/stresses then cause the cracks in the tile right over those seams.

There's still a bit of grout haze showing in a few of the photos. Depending on how well they did the grout cleanup, a hard buffing with a microfiber might take care of that. If it's worse, there are commercial products to help with that.

I hope you get it resolved to your satisfaction.

I could not see the cracked tile from your pics....sorry.

One thing I ALWAYS DO is, when there is a tiled wall over a tub....never grout that gap. It should ALWAYS be a pure silicone caulk. Also, you want to fill the tub at least 3/4 full of water....caulk the gap and leave it alone for at least a day...or longer. Doing this, the caulk will "cure" in the expanded mode. This is (IMHO) critical to preventing the caulk from pulling away when the tub is in use.....

The crack is on the right side of the large tile in the first pic. Not very obvious but it's there. I always hear that one should never grout when the planes change and it is good to know to caulk when the tub is 3/4 full. But now he already grouted both the gaps between tiles and gaps between tiles and tub.

What can I ask them to do at this point? The owner's answer "Grout shrinking like that is common, especially if it is non-sanded.  We have to put a second coat on it.  We also caulk the inside corners with colored caulk to match the grout". Will these be possible without cleaning up the previous grout? I just don't know whether I asked for too much or whatever he suggests is workable without redoing the whole thing. Thanks.

You will need to communicate with him about replacing the offending tile. As to your other question, they need to dig out the unsanded grout and silicone the areas as I earlier directed. ANY "change-of-plane" needs to be caulked....period. All materials WILL move relative to temperature and humidity and "loading" as in filling your tub and adding your weight. Just the way things work, eh? Keep us posted.........

The Redguard is no where near thick enough, all changes of plane should be caulked, the tile cracking is not acceptable.

The fastener schedule was not followed for the backer board. The backer board and tub flange interface is incorrectly done. The Redguard was applied incorrectly. The use of unsanded grout is questionable. The use of any grout at change of planes is flat out wrong.

@Verbo I think the tub might have a double flange. If so then the CBU to tub junction might be ok. There is definitely a lot of flange showing at the CBU stage which would only be right with a double flange tub or a cast iron tub.

It is a Kohler tub with apron on all three sides.

Elmbrook is a single flange acrylic tub so it looks like the wall board is not installed to Kohler's specifications.

Alas, feels like I am driving on the road after freezing temperature, potholes one after another. Don't know how many I hit and how many are yet to come.

"The Redguard is no where near thick enough..."

Waterproofing is optional according to USG.

FYI @neroli USG don't build showers and a waterproof barrier either behind or in front of your USG board is almost certainly required by your local building codes...Joe is a bit of a comedian.