Is this a current or previous guest? Is you main concern a financial one or boundaries i.e., the guest's guest stayed late as such the guest should paid extra or that the person was in the property past your permitted time frame?
I note your house rule state 'You are welcome to have guests visit but they need to leave at or before 11pm. There are no overnight guests here.’ Was this rule in place when the guest booked your accommodation? If so, then you are within your rights to tell your guest that her guest has to leave the property as this particular subject is clear in your house rules; if there are any issues as a result of this which I would hope not, Airbnb will back you if a guest breaches your house rules.
However, if it is a new rule you have added perhaps as an afterthought following this situation, this is how I would handle it. If she has one/ two days left to stay -I would hope she has taken heed from the conversation we already had and just observe things to see if it happens again. If longer, I would have a word after the guest has left, briefly explaining briefly my position regarding future guest and that if she does wish to have guest stay after 11pm,you would regard it as an over-night stay therefore an additional fee would apply. In addition, you would expect her to agree this in advance with you and if convenient, then you will make the necessary amendments including extra payments to reflect this on the Airbnb platform which she can agree to and then you are both covered by Airbnb host/ guest protection.
If she has already left, you could mention in the review that following house rules was a bit if a challenge in particular with non-authorised additional guest.
Wish you all the best,
Ama
