Yes, there are DIY ways to improve the appearance of the grout and some of the etching. And I can understand you not wanting to deal with this bozo. Only you can decide the point at which conflict avoidance tips the scales on your personal cost/benefit ratio.

That said, ask yourself.... if this is what I can see, and the tiler is defending it, wth is underneath it all that I can't see? This could be way more than a cosmetic fix. Aside from the sloppiness of the grout, I'd also be concerned with the grout at the ceiling line. I presume he also grouted the shower floor joints at the wall, too. Hard to tell what the vertical joint is from the pic. The point is, wherever there's a change of plane, it should be caulked. None of this suggests your tiler would have been any more conscientious with water barriers, shower pans, etc. that are now tiled in.

Before you touch anything, be very sure you're preserving your rights along the way. In many locales, you have to give the contractor a chance to fix things before making any claims against his insurance, initiating legal action, withholding money, or filing a complaint with a licensing body.

There won't be any claiming anything against his insurance unless he caused property damage or bodily injury to someone. Contractors and/or their respective companies are personally responsible for shoddy workmanship. It's pretty simple; he completes the job to the owner's satisfaction, or he refunds her money voluntarily or by court order.

Yes, I believe everything was grouted. The joints do not have the plasticity of silicone or acrylic. After a closer look, I realized the uneven line at the ceiling is a paint defect. I will also need to remove the grout from the base boards:

Instead of spending more money trying to fix the problems myself, it is probably better to mention these things to the contractor. I just hope he doesn't make things worse.

The grout lines themselves, do not look even and some of the spacing looks a bit off. The tile itself is very nice, but, the workmanship is not.

As stated above, a "contractor" who takes that little pride in providing and then defends that poor of work will have almost certainly skimped on what cannot be seen. What waterproofing was used? Shower pan? Was the shower pan flood tested? And all changes in plain get caulked not grouted.

He did a mud job and membrane for the shower floor and used cement backer board for the walls. I do not believe he did a flood test because this requires water to remain overnight in the base of the shower, correct? I checked on their progress daily and never saw water anywhere except in the reservoir for the tile saw.

Tile, grout, and backer board, are not waterproof.

What you describe, and what you show, say that you’re dealing with a cheap hack. Not a professional tile contractor. That’s your typical $1.98 Craigslist “tile guy” work. It’ll last long enough to allow him to cash the check.

That needs a complete gut and do over. From someone who is not a $1.98 Craigslist hack.

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