My mom was hospitalized for rehab at a point and while she was there, her insurance denied several appeals to pay for it and now she is left with a 20k+ bill for her treatment. When she was still in the hospital, she signed a paper (not sure what, she doesn't remember) saying she was liable for the bill. At the time, she was under medication and stated that she was not in her right mind.

Any tips on where to start to get this taken care of?