Did your son and his girlfriend make loud noise at the listing causing the host to ask Airbnb to cancel his booking. If so he should have left the property . @Christopher1786
What exactly are suggesting are ways in which Airbnb could have prevented the host from allegedly attacking your son and girlfriend?
if your son and his girlfriend were being excessively loud at the listing Airbnb can suspend his profile and not allow him to use the platform.
Why is the refund relevant ?