Hello @Steven0
All the contact details for Airbnb are in the Contact Airbnb post at the top of the forum you are posting in.
I would message the guest back on the platform and say if he needs to cancel he needs to call Airbnb directly. And that any refund due, will be handled by Airbnb in line with the cancellation policy he booked under.
It is easy for your guest to reset his password if he has forgotten his account details.
Do not cancel on behalf of the guest. You will incur penalties.