She was stuck on a train because there was an avalanche blocking the tracks. The day of the reservation (after check-in), I was notified of this. She had a 7 night reservation, but only stayed 6 nights. I just received a "request for money" for the night she missed. I'm happy to give her a refund for one night, but how does this work officially through Airbnb?
You are not responsible for the weather or nature disasters, so not obliged to refund the guest. Maybe her travel inssurance can help her out. But if you want to refund her, then use the resolution centre. The reservation links to it with options "receive or sent money", more: