This happened to me and still makes me wonder.
I am going out with a women with two kids. One is 12 and the other is 15. It was coming up to the fourth of July I asked what are they up to and she said she didn’t have plan and was going to stay home. So I asked how about finding something or do something and she agreed. So I found a town BBQ in the next town. When I showed up to go her oldest daughter didn’t want to go, so I said leave her but my GF insisted she comes. Well when we got to the BBQ the daughter refused to get out of the car. Actually she and her mother had a chat for a half a hour. So after the BBQ we went to a ice cream parlor and the daughter didn’t want ice cream but once again the mother pushed it on her. When we got back to her house the daughter went into her room. Her mother went into the room and talked for a while. A little while later my GF told me I had to leave because her daughter was in a bad.
At this point I am wondering who is the adult in her house.
Speaking as as someone who is a step dad and has lived this... my advise is get used to it or get going! If you're going to date someone with kids... either pre-teen or teen... you're going to see this. There is a very good book you might want to read called 'stay out of my life, but first, can you give me and cheryl a ride to the mall'. It's by a Dr. Wolf and is an incredible insight into the mind of the teenager. It will help any parent or would be parent with the difficulties of dealing with and raising a teenager!
My 2 kids are wonderful and the lights of my life (they are steps) and I would not trade one second of the time I've spent both good and bad!
You might buy the book and read it with your girlfriend... we all have challenges dealing with teenagers and it's especially difficult to do alone. It can be very easy to allow yourself to get dragged into their world and it's easy to wonder who the 'adult' is... Teen's don't come with a manual... don't forget.. your Girlfriend would probably have enjoyed the day more if her daugter had behaved differently.. certainly in front of you. All I can say is.. it's not easy. Some people are better at it than others or catch on more quickly.
In the end it boils down to this... do you want to be a bystander or involved? Even if all you do is to provide support... are you in or out?
Good Luck