It would have been much better for you to tackle this when the paying guest arrived and confirmed with her that she had only booked and paid for one, so her partner who wasn't on the booking wouldn't be able to stay. @Tom-and-Hayley0
As you weren't comfortable with the partner because of his behaviour you could have said that as her plans have changed it would be better for her to find an alternative place to stay.
Now you have let them both check in it is more difficult.
What outcome do you want? If you don't want them to stay contact Airbnb say your guest has bought an additional guest to stay who isn't on the booking and his behaviour is making you feel uncomfortable.
Your choice is a) contact the guest and confirm only she can stay b) let them both stay only if she accepts the additional guest charge c) if she wants both people to stay and you don't want this ask Airbnb to cancel