I’m not really understanding the circumstances here, though. What does, they wouldn’t let her check in for her return flight because the “flight arrived too late” mean?
We’ll need to wait for the OP to come back and explain. Given the option provided for having to come back the following day, my interpretation is that “they arrived too late at the airport and thus missed the flight.” It could have been too late to meet the check-in window timing requirements or totally missing the flight.