@Monica4The guest has 48 hours after booking to cancel the reservation penalty-free. After that, Airbnb keeps the service fee and the cancellation policy is applied based on whatever has been selected by the host.
When you book a property, it's explained pretty clearly. The listing will state "Free cancellation for 48 hours": it doesn't just say Free cancellation with no details. I use Airbnb as a guest as well, and it is clearly worded.
If she hasn't already, I would suggest that she gives Airbnb a call and pleads her case. She may get a sympathetic rep, especially if she has used Airbnb a lot.