Hi @Christine498 I see that the max. occupancy, if I try to book, is 1 person. However, it looks like you do not have thorough house rules to prevent this kind of situation. Unfortunately, some people do not have common sense/do not care about following the contract and try to break it.
I suggest that you update your house rules to be more detailed with things that are important to you such as not allowing any extra guest/visitor. Spend some more time in the forum to learn from other host's experience to help you improve your listing.
Personally, this is the first house rules I have in ALL of my listings (yes, including the entire apt that I rent out) > • No unannounced extra person, visitor, or guest other than you is authorized in the unit at any time day/night. Failure to comply with this agreement will cost our agreement without refund.
With having that in the house rules, it helps me/makes it easier for me to enforce it because guests are REQUIRED to agree and follow house rules.
For now, it is up to you on what you want to do with the situation. You have the rights to ask her to leave because she is not an authorized guest in the reservation. You can also let her stay and ask for an extra person guest fee to the guy, but keep in mind that you do not have any of this information (extra fee in the listing since you only allow 1 person).
If I were you, I would ask her to leave immediately and send a message to the guy using the airbnb system summarizing what happened and that what he did was not allowed as the reservation is only for 1 person.