Technically, your guest/guest's friend is violating the Terms and Conditions. Is this person also travelling with the named guest? If not, I would be a little concerned.
Sometimes this isn't an issue. I have had several people book the place on behalf of their less technically-minded parents for example. I know that *technically* one shouldn't book on behalf of a third party, but I turn a blind eye in these cases. (That means I am also exposing myself to risk if I needed to make a claim on Airbnb's insurance too, I guess!)
If you are genuinely concerned, you could reach out to Customer Service and explain that the guest is not responding and that the 'guest' isn't actually the guest. They could cancel the booking without penalty to you.