If they ban your guest it's because you want it
The thing goes like this.
You are an adult traveler and you have made a reservation for two pax in one double room. They can't ban her.
If you bring a second guest during the same day / night, that's another story, hotel has the right to charge extra or make a new reservation.
If your reservation is a single room for one person, then you made a bad choice.
* * * * * *.
I remember a hotel in Chile suggested that I could not "enter the hotel with a pro*t*ute" and I got mad and said "are you telling my girlfriend is a pr*ti**te?
When the girl left the hotel one hour later, I got a phonecall from the reception "Hey sir, your girlfriend just left the hotel" and I replied "oh yeah, we have argued" LOL.