Big Girls Don't Cry (movie, 2002)

Kati and Steffi have been best friends since they were six. Now they're both 17 and enjoying the ups and downs of becoming adults together. While Kati's parents are religious and conservative, but argue constantly, Steffi comes from what appears to be a harmonious and liberal family. But the girls discover how quickly their attractive world can fall apart when, at a hip nightclub, they happen to see Steffi's father in the arms of another woman. Steffi is shocked, her trust in her small perfect world shaken to the core. She can only think of one thing: revenge.