If this hardwood flooring contractor also installed baseboards after he finished installing the floor, then I don't see how the difference in floor height WOULDN'T have come to his attention. He would have noticed that for sure when installing the the baseboards at that transition. He couldn't have missed noticing that the baseboard elevation that works well on one side of the transition DOESN'T work on the other side of that transition, and would have had to figure out what to do with the baseboard in that transtion zone.

It seems to me that the best fix would have been to add an additional layer of underlayment to the area that was at the lower elevation first, and then transition to the floor at the higher elevation.

Johnsonite makes a stiff rubber material specifically for this purpose. It comes in 12 inch wide strips 4 feet long that go from effectively zero thickness on one side to 1/8 inch, 1/4 inch, 3/8 inch and 1/2 inch thickness on the other. The following image shows how the Johnsonite subfloor leveller system can be installed:



The green sheet flooring is thinner than the grey flooring, and so the tapered rubber strip is installed to transition between the two finished floor heights.

In your case, a better option would have been to add a layer of 1/2 inch underlayment to the part of the floor that was at the lower elevation, and then use these tapered strips to fill in the transition between the new underlayment and the higher elevation floor. If you use two pieces of the 0 to 3/8 inch transition and a 6 inch wide piece of 3/8 inch underlayment, you can go from 0 to 9/16 inches difference in floor height in a foot and a half wide transition. From what you're saying, that would have worked out well on your floor.

Maybe they started installing the hardwood flooring on the part of the floor at the lower elevation first, and by the time they encountered the change in floor elevation, part of the floor had already been covered in hardwood, and fixing things at that point would have meant tearing out the hardwood they'd already installed. Maybe this was a new foreman leading up this job, and he just didn't know what to do about the problem once it was staring him in the face, so he just decided to finish the job and hope you'd just learn to live with it.

Still, if this was a professional hardwood flooring contractor, then he should have been on the look out for things that could screw up the job, LIKE changes in floor elevation. After all, this wasn't his first hardwood floor installation, and he would have realized that something like that would need to be corrected BEFORE he starts installing the hardwood. So, I agree with the others that the contractor should have noticed and addressed this problem before installing the hardwood.

Have you already paid the contractor? Either way, I'd maybe bring this to the attention of the top dog at that company. Even though the job is finished, they should acknowledge that it was a mistake on their part to install the hardwood without discussing the matter with you first, and they may be willing to fix the floor at no or very little cost to you.