Jo this is a third party booking and therefore it isn't covered. I personnaly don't accept third party bookings as indicated on my Listing and would not accept a request to do so from another host.
You were not made aware of this beforehand and only accepted the host, not the son. You could contact the host and ask her to cancel her Request as it is a third party booking but she might not agree to do so.
If you don't wish to have him stay you need to contact Customer Service to cancel the Booking on their end. Don't cancel on your end.
Best wishes,
Yvonne