What legal rights does my father have when the nursing staff is billing him $25,000?

My father is 82 years old and was put in a skill nursing home for rehabilitaton from brain surgery for a blood clot he had . He has the beginning of Alhzimers and Demansia. The nursing failed to notified him that his benefits had ran out 30 days prior to his discharge. We his kids or my mother were never told that his insurance would no longer covered his stay there. As a matther of fact his insurance send my mother a letter stating that the nursing home had contacted them for the first time 3 days prior to his discharge. My mother and i (daughter) and along with my siblings met with the nursing staff the month prior from the month he was released and one of my questions to them was how much longer my dad had coverage. Their answer was "dont worry, we will let you know in advance once his benefits are getting close to running out." We had express to them that this was our first time dealing with such situation and to please let us know what we needed to do for my dad. I mentioned several times during his stay if i needed to contact his insurance myself to please let me know and every time they said don't worry everything is fine . Now they are billing my father for $25,000.00. That includes for the month that medicare did not covered also for abulance services and for the skill nursing home. I am sure he will be billed for more down the road. I need to know how my parents can protect themselfs?, what rights do they have?. My mother doesn't read or speak english and the nursing facility had her sign paperwork in english that she was responsible for the charges. Also the form she signed has a section for witness which there was no witness at the time she signed and its not even dated. As far as know she could of signed it the day he was admitted! who knows when. I've been talking back and forth with the nursing home but they keep telling me that it was my dads responsibility to contact his insurance. But as i had mentioned before no one ever told us that. My dad is 82 years old as you know his memory is not well, my mom is ready to have a nervous breakdown and she is 72 yrs old. she is worry they are gonna come and take her home of 45 years. My dad is now being cared by my sister because we are scred of putting him in a skill nursing place again. My dad just recently got out of the hospital again after being there for 3 days from a terrible fall. Again they wanted him to go to a nursing home instead we tooking him home and we are trying to get as much care as we can for him there. He has Medicare plan A & B and private insurance (Blue Cross Blue Shields government program a PPO plan) . Please advise!