@Kristie0
You could confront the guest in person about your sense that she was having someone else (presumably her boyfriend) over in violation of house rules,--- but it is very difficult, if you didnt' actually SEE the boyfriend or other person there, to do much about it, officially, such as evicting the guest for violating rules, using resolution center to seek payment for additional guest, or writing a review in which you state the guest had someone over in violation of your rules.
Hosts are going to continually have this problem -- that hosts who are not there to be able to observe first hand who is in their home, are going to be victimized by dishonest guests who sneak in additional people. See this post which goes into that issue:
https://community.airbnb.com/t5/General-Hosting/Number-of-Guests-Booking-Suggestion-for-Airbnb/m-p/2...
Unfortunately, there seem to be a large number of dishonest guests, as this problem you are having has been quite a common complaint of hosts, actually for some time. I have read about this problem for the last 2 years in the host group forums -- it seemed to me that the issue was brought up at least once a week by hosts. THe problem of guests bringing in extra people/visitors, who are not on the reservation, is I think the MOST common problem that hosts have who offer entire apartment/home listings where they dont' also live.
Thus hosts need to decide what they will do about this issue. Some hosts may "drop by" unannounced, to see who is at their home. Some hosts will ask their neighbors to watch and report on who goes into their place. Some hosts will put up surveillance cameras outside the listing so they can see who comes to the front door. Some hosts will increase their nightly rate if they feel that they continually are hosting more people than the reservations have been booked for, and want compensation for the use of their space. Some hosts will state in house rules that if guests bring in people who are not included on the reservation, their reservation is subject to immediate termination and forfeiture of all fees paid, as a way to try to deter this dishonesty. Some hosts will throw up their hands and decide there is little they can do about dishonest guests. Except maybe -- not rent to them again ---