Hi @Lukas209 🙂
If a guest decides to leave early the guest must cancel and she will get refunded according to the cancellation policy you got and she agreed on when she booked. You don't have to do a thing it all happens automatically when she cancels. Tell her that it is how it works and that airbnb takes care of all money transactions etc.
Whenever I have a guest who cancels I tell the guest that in case some of the days are rebooked I will gladly refund those days but as she booked and blocked the dates as noone else could book the cancellation policy always kicks in.
An important thing is that she can sent you a request to cancel. NEVER accept such a request as it will then look like you cancelled. She will have to press the cancellation button all by herself and you will receive an email that she cancelled and the reservation will get updated saying she cancelled.
When she cancels your calender will open up and you can of course refund her whatever you like via the reservation after the cancellation has been made.