Excellent Allegory about different kinds of abusive relationship
Plot holes aside, this movie did a great job of capturing the pain of being in an abusive relationship.
1. She feels paranoid all the time. Even after she ended the relationship, she feels he's watching her (in this case literally)
2. He isolates her from her sister with that email. A lot of abusive partners isolate their SO from their family and close ones.
3. Her friends (the cop mainly) start to think she's the crazy ones. A lot of abusers are charming to the outside world and make the victim look bad.