Can you help me to find this book?

I don't remember what it was called. I do remember that the cover was orange though. It's about a girl who starts sleeping with this boy, and then becomes friends with his girlfriend. The girlfriend finds out and so does her best friend. The best friend gets upset (because he hate the guy) and he seems to be secretly in love with the main character. I also remember that the main character and her best friend had this kind of tree house like hideout that one of their fathers built for them. Thanks!