These gaps are pretty big. Looks like it was improperly acclimated. Moisture content needs to be within 2% of the subfloor. The staggered joints should be at least 8" for your 4" flooring. This would help keep the joints tighter, compared to the 4" stagger, which is below the minimum 6". You can trowel fill, if, the desire is not to remove it and start all over.

What region are you in? How long ago was the floor put in? Was the home properly conditioned before acclimating?

The image with the hand. Shows a board that is milled undersize on the width. 1/8" gaps is too big. These undersized boards should be replaced. You can go around and measure the width wherever there is a large gap to make sure the width are the same, repair as needed. Then sand, trowel fill, sand, fill again and finish.

When we replace unfinished boards, regular wood glue is applied to the tongue and groove. And then it is faced nailed. Nail holes just gets filled.

Who is on the hook for replacing the flooring?

Another alternative is to purchase 6" flooring and have each board custom fitted. Use an experienced installer, they can rip the board to width and groove the flooring and then install it. Each board would be a custom fit.

I don't agree with not taking him to court. This man did a lousy job, and should be outed. This won't reflect on you or your business, except that you will be saving other people from this nightmare. Don't let him slide.

As for me...I love my hardwood floors, and it would irk me every single day to look at your floors. I would have an installer tear it up and start over, take the first installer to court and have him pay for any boards that can't be reused, and the labor you paid him.

One other thought...when I've hired contractors, I have never paid up front. Payments are usually split into 2 or 3 payments, one up front, one about halfway through, with the final payment not given until the job is finished, and finished properly. Any contractor that wants to be paid in full before the work is finished would never be hired by me. If you had set this up like that, you would have only been out your first payment before you realized this was going south and stopped work.

Is the installer the same as the seller of the flooring? It looks like it is unfinished wood - is that right? What is the brand of the flooring?

How big are the biggest gaps vs. average gaps? Do you expect the humidity to be higher during other seasons. closing the gaps somewhat? Does the floor move at all under traffic?

I suspect that the floor has a little bow in it. This can be caused by poor technique, unlevel subfloor or a variation in board widths. Sometimes a board can get a little crooked and the butt end will stick out a further than the next board. When the next row is laid it will get misaligned if the installer isn't paying attention.

If the floor feels sound and you don't expect it to swell with seasonal changes I'd be inclined to sand and fill. If there any boards with consistent gaps over 1/8" I might consider board replacement with 5" wood ripped down to fit tight, keeping in mind that this eliminates most of the tongues and grooves that meet adjacent boards.