I'm pretty sure she'd have to be legally emancipated to stay against her parents' will. Those not doing anything could have seen her acting up in the store and knew she was just upset with her parents. Yup, say something and call the cops.
EagleKS, yup that woman is damn lucky you showed restraint. Glad it worked out for you. She probably still didn't get the message since she didn't have to talk to the cops about what she did.