If paying out of pocket at SNF for more than 60 days, does the benefit period ever restart?

My father in law has been in either a hospital, rehab hospital, or SNF since May of 2017 due to a traumatic brain injury from a fall. He developed a bed sore and that finally healed after 4 surgeries in December. He started rehab at an SNF in January of 2018 and was recently sent to a rehab hospital for more intensive therapy. We were notified yesterday that his days have run out at the rehab hospital and he has to go back to the SNF. Our 100 days there ran out in December and my mother in law has been paying out of pocket for 3 months. Originally, we were told his 100 days would reset, but I don't think that is the case. Any guidance would be very appreciated.