Hello Jeannette
I am not sure how she was able to cancel a booking when she already stayed.
I hope you followed up your conversation with her on Airbnb so there is a record.
In your situation I would call Airbnb and explain what happened. And ask them to follow up with the guest on the extra person charge.
Her cancelling the booking shouldn't make any difference.
Make sure you leave an honest review.
If you have a remote listing, install CCTV at your entrances so you can see who is coming and going from your property