@Adrian525 you can cancel even if they are not under the influence of anything. The person that made tge reservation must stay there, otherwise it is a third-party booking and it is against Airbnb policy. You can call Airbnb and have them cancel the reservation penalty free.
If the guest who checked in, is not the same person this would be considered a third party booking which is a violation of Airbnb's terms of service. I would advise reaching out to Airbnb immediately so they can assist in real time while the reservation is ongoing.