THANK YOU for a speedy reply!
I've written to the mother of the guest, who does not have access to his account - and so she cannot cancel. I've let her know that she has to get in touch with Airbnb herself (and probably with her/his insurance company too). I read up on the cancellation policies, but this was sort of a special case. Be that as it may, I've done what I could.
The links provided will not travel through the message board on a non-accepted request, so I suggested she went looking herself on the community sites.
Thanks again, Pernille
(I do however miss the excellent drop-down menu that used to be... i had a case in 2012 (?) where my guest-to-be was being very offensive and rude, and I needed to get in touch with Airbnb pronto. The drop-down menu let me chose my state of concern, ranging from - if I remember correctly, "a little worried" to "completly panicked!" and the responstime from Airbnb was set accordingly).
Pernille