Change in Reservation asking for more money & not refunding

[Roger18]

Hi my names Roger, i have recently joined Airbnb, so i'm learning as i go.

I have booked my house out to a member for 3 nights, they have come back after confirming & said they cant make the first night, so i went in & changed the reservation to 2 nights, the site have come back to her & asked her for more money, also have not offered a refund for the first night she is going to miss. any info on this would be very appreciated. Does she get her refund after the stay ? is the extra a penalty ?

Thanks Roger