@Daria175 If the guest booked before 14 March, he would have been eligible for a refund direct from Airbnb under Extenuating Circumstances. But if you received the booking more recently, he doesn't have a leg to stand on: everyone has known for months that we're in a global pandemic with significant odds of our plans being thwarted by travel restrictions, and it's the traveler's own responsibility to check cancellation policies and follow the indicated procedures.
What happened in this situation does not constitute an "emergency"; for anyone who hasn't been living under a rock, it was a totally foreseeable inconvenience.