@Zoe127 Did you call Airbnb support? Here is the phone number for Airbnb support in Australia
The other way to reach Airbnb support is to send message through twitter. In your case, a long-term cancellation policy should be applied after the guest checked in and wanted to cancel unless there was an extenuating circumstance. But it does need the guest to provide support documentation.
@Zoe127 your regular strict policy is not applied here. He booked for over 28 nights so the long term cancellation policy should be applied.
I understand and feel sorry for your frustration :( Unfortunatelly I've seen too many posts about Airbnb not honoring its own policies and TOS and the host is always the one at lost so I am not surprised.
@Zoe127 It sounds like the guest reported either a Travel Issue or an Extenuating Circumstance when they contacted Airbnb. Either way, it should have been disclosed to you, and your calendar should have been opened. But Airbnb sure is mishandling a lot of issues these days...