Surgery approved, then denied.

My mom, whose insurance (Cigna/Aultcare) had initially approved her surgery last month*, is now denying it. Cigna told my mom the surgery was approved so she set the date with the surgeon. Now that the surgery is a week away, her surgeon called to confirm with Cigna about the approval. Cigna/Aultcare/whoever (sorry y’all I’m just upset) has now backpedaled saying the surgery has been denied because they “don’t cover foot care”. It’s a reconstructive surgery for fallen arches and if she isn’t able to get this surgery done on both feet, she will continue to be in chronic pain and eventually unable to walk. We have never once been given a list of what is included or excluded under my dad’s employer’s insurance plan other than “we don’t cover vision or dental”. Cigna blames the company for this and told my mom to ask the company what’s included and what isn’t, but we feel Cigna should have that information too. My mom has reached out to the HR department about this issue, but my goal here is to understand 1, if it is LEGAL for health insurance companies to approve and then deny coverage (mom would not have made an appointment for the surgery if it wasn’t covered/had been denied!!), and 2, is there anything she can do in this situation? The surgery would cost tens of thousands out of pocket. The look on her face when she told me all of this broke my heart and I’m angry they told her it was all set and ready to go, and then went back on it.

For clarification, I believe she was only told verbally over the phone that the surgery was covered and approved. From what I understand she does not have any written statement saying as much unfortunately, which I think would have helped greatly. I will confirm this information with her the next time we talk (soon). Thank you for any and all information.

*EDIT: Corrected my facts here