Pre-existing conditions.

[Ms. Toad]

7. If the surgery happened before the ACA

the policy could well have had an exclusion for all treatment related to the condition. Typically you were just rejected for insurance entirely, but a few companies issued coverage but excluded treatment for any conditions that were pre-existing. Other policies had a maximum out-of-pocket. Once you hit that, the company was not under any obligation to pay more.

If the policy issued after the ACA, and it is a compliant policy, the care was provided within the terms of the policy, it should be covered at some rate (there might be a large (~$10,000) out of pocket payment required before insurance kicked in - but medically necessary treatment after that would be covered.

(It could have been a grandfathered policy, or it could be short-term insurance - both of which are choices (at least nominally), and are not required to be compliant)

You don't have enough details to really get a picture of what is going on.