Maybe he had renter's insurance or something, or some other form of liability insurance? (I don't know how this works in the UK).
If I were the lessor/host, I'd demand Airbnb pay 100% and then let them go after whichever other parties they want later. The host should not be the one on the hook for any of this, including float for however long reimbursement takes.
(If I were Airbnb I'd probably sue the guest; clearly there was some level of organization if they brought in an audio system and had an event. Even if it cost them $20k in legal to go after a $10k judgment it is probably worth the precedent.)