Hello @Frances27
As @Helen3 explained, your usual "strict" short term policy no longer applies when you have a stay of over 28 days as your cancellation terms transfer automatically to Airbnb's long term cancellation terms - 30 days notice.
Your guest was able to shorten his stay without needing your approval because he has given you more than 30 days notice.
So my interpretation of what the Customer Service person said to you is that your guest owes you the time already spent in your place from the time he notified you of the change - 47 days + 30 days notice = total owed 77 days.
To check this out here is a link that details cancellations/alterations to long term rentals:
https://www.airbnb.co.uk/help/article/447
Joëlle