I am a medical biller in NJ for the last 27yrs. I also was my parents POA. I am quite familiar with this sort of billing and most Rehab facilities participate in Medicare. (which means accepts what medicare pays as payment in full) So there should not be balances for services, except if there is additional days not covered by Medicare which is then paid by the patient if they do not have a supplimental plan. Hopefully I am understanding your post correctly. I would most definitely contact the billing department of the facility and discuss this further and not wait for a surprise bill. As for the account being past due, it is only past due 90 days from the first bill. Make sure you document whom you spoke with and your conversation and confirm addresses and phone numbers. Checking prior to paying bills is for sure a plus. This is for sure a problem, insurance fraud as well. Overcharging and charging for services not performed as well.
Best of luck and hope I helped somewhat.