Does inpatient rehab facility have to notify patient if insurance changes halfway through?

My husband was at a rehab facility and they told him they would notifiy him immediately if there was any issues or change in his coverage. Halfway though insurance changes, but they did not notify him. They said they though insurance has sent a letter expaining it to him. Now they are saying they are sorry there was confusion and we were not notified, but we owe them an extra $4000. Since he was inpatient and under there care, were they legally responsible to tell him about the change? - Is this your question? Add additional information