The insurance information for both the primary group plan and Medicare should have been provided to the rehab facility when admitted. The provider should have been billing both the primary and secondary (Medicare) all along. In some cases, the primary group plan forwards the claim information to Medicare so the primary plan also needs to know she is Medicare eligible.
The provider may indeed be billing Medicare and it may be that Medicare is currently paying zero as secondary. You need to contact the rehab facility and verify this is the case so that when the primary benefits are exhausted, Medicare will have a record of the admission and start paying under Coordination of Benefits (COB).
Also, if the rehab facility is classified as a Skilled Nursing Facility (SNF) by Medicare, there are additional requirements that had to be satisfied before Medicare pays a SNF. The SNF should be able to tell you if those requirements were met (3-day hospital stay prior to SNF admission) or if the SNF qualifies for a waiver of those requirements.
