When someone who isn’t on the reservation reaches out, it’s best to protect yourself by only communicating with the verified guest. Politely let them know that, for safety and policy reasons, all communication must go through the person who made the booking or someone listed on the reservation. Here's a message you can copy and paste if you'd like:
Hi there,
Thanks for reaching out. For safety and Airbnb policy reasons, I can only discuss reservation details or anything related to the property with the person listed on the booking. If you'd like to be added to the reservation, please ask your friend (the primary guest) to update the reservation accordingly. Thanks for your understanding!