@Ted403 The host should know about the third-party rule. It’s in the TOC. So feigning ignorance does not, and will not, hold mustard.
So I believe host is responsible for any damage to their place by a non-registered guest. And if the guest was a family friend, why wasn’t the guest able to get in touch with the family friend regarding any problems. So, the inconvenience of the guest may be a non-issue as far as airbnb is concerned but I’m not 100% sure.
It’s very important to be registered on airbnb for transparency and trust for all guests and hosts. It only takes about 10 minutes to signup so it behooves me that anyone would take a chance of third-party bookings if you’re not a business booking for its employees.