If somebody just doesn't feel like "messing with the situation" and goes home without giving the carrier a chance to deal with her individual situation, including possibly creative routings to get her on her way to her destination that night or a confirmed seat on the first flight out in the morning with possibly some nice gesture of apology, then I'm inclined to be somewhat sympathetic toward the airline. Your sister should be due a refund if and only if her fare was refundable; if it was non-refundable, she's due the fare basis toward future ticketing. That's assuming she didn't simply show up at the airport Saturday morning, explain the situation, and get a standby on the first flight out.