Essentially identical story in my family. And to address the sibling comment's question in my case : the hospital has a _recording_ of the insurance company rep on a phone call stating that the procedure was covered. However it appears this achieves nothing because the insurance company can use their magic powers of "pound sand suckers" and we had to pay anyway. Hospital denies any responsibility because they called insurance and were told they had coverage, and they kept a recording of said call. The fact that providers routinely record their calls to insurance companies tells you something in itself.

Just out of curiosity, because I often have this fear, did your the staff have any documentation or make assurances that he was a "participating provider" with your plan (i.e. not just saying they "will bill the insurance company")? Or did your plan not offer comprehensive coverage?