Keep in mind the following
They should have travel insurance in the event of delays caused by cancelled flights, illness, etc..
They can approach AirBnB if they have extenuating circumstances - eg close family member is very ill, etc..
Then depending on how you feel, how far away the booking is and whether or not you think you can resell the rooms - you can either give a full refund, say you will refund the nights you can sell, etc...
Also subject of course to your cancellation policy.
Often helps to put yourself in their shoes - how would you like/expect to be treated.