@Janine---Micha0 Hi Michael, This something that comes up a lot on this forum.
I would message the guest through your Airbnb messages saying..." You have booked and been accepted as a booking of 1-2 guests, and any further guests will not be covered under the umbrella of Airbnb's protection. I want to be as helpful to you as possible but this would place me at considerable risk and unfortunately I can't accept that additional risk. I am sorry but, It's the same as booking a room in a hotel, we have to work within the rules"!
That way Michael you are puting the 'guilt' back on her instead of making you look the bad guy, and your review should not suffer, unless she has other issues.
See how you go, I hope it works for you.......cheers.....Rob