@Monif0
The 'messaging system' is the normal means we communicate, Hosts & Guests, i.e. 'Inbox'.
It's not private, rather part of Airbnb website, and so can be monitored or checked at any time. It's also why Airbnb encourage both parties to communicate on their messaging system in case of any disputes.
------
In all 3 of my cases there was record of the 3rd Party infringement on the messaging system, which made it very easy for the Case Manager to take action.
------
In your case, I understand the Booker did not mention the other 2 at any time on the messaging system.
If so, failing direct evidence, Airbnb can peruse the Reviews.
From your comment here: ' I'm not reviewing the person who booked but rather the couple who stayed so I wrote a review to that affect ' - I adduced that you clearly stated the people who stayed were other than the Booker. -
However, you actually wrote: 'Quiet, clean and courteous guests. No issues, thanks for staying with us!' - It rather distinctly creates the impression the Booker was one of a plurality.
Consequently, it seems there is no written record of the 3rd Party Booking.
If that's the case, I can best suggest you write to the Booker a polite but firm message:
both explaining & spelling out the situation.
This was your first booking and she threw you off balance. Both of you should have been familiar with T&Cs, and more easily the notation in the 'HELP' facility.
Type in 'Who can book trips on behalf of someone else?
It stipulates two types: Business & Personal travel.
The latter personal travel: 'We require Airbnb reservations booked for personal travel to be booked by the person who's going to stay at the listing.'
Consequently, kindly ask her to acknowledge that she had no intention to stay, and therefore should not have made the booking, and in turn not entitled to review.
Further explain the consequences of her action in respect of Reviews.
According to you, 2* does not chime with the experience of the two Guests.
Moreover, you can spell out the problem such a low star review can generate from Airbnb.
Summarily ask her politely to contact Airbnb to acknowledge her own infringement of T&Cs and so request the removal of her Review and Star Ratings.
Best of course if she acknowledges and contacts Airbnb.
Per chance she responds, and acknowledges the situation, but declines to contact Airbnb, you can then do so yourself citing her acknowledgement.
If she does not respond, so that you continue not to have a written proof, you can at least ask Airbnb to contact her with your renewed account on the messaging system to see if they can elicit a reply from her.
------