Two almost opposite suggestions:
Since your listing uses the Flexible cancellation policy, you'll get nothing if the guest cancels. So, I have to ask... given that you've listed a 1-bedroom space, how much would it really matter to you if 2 guests shared the space? Could you live with that?
On the other hand, the guest knew (or should have known) what they were booking, and that was a 1-person-only space. So, hold your ground. Do NOT cancel the booking yourself, as AirBnB will ding you in many ways for that. Instead, hold your ground (with strong, clear, brief messages to the guest, for the record) and let the guest cancel if they insist on bringing a friend. If they don't cancel and then do show up with the second person, contact AirBnB and have them kick the guest out for violations of your listed policies.
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