A psychological thriller where girls play pretend

So my friend watched this film and only remembers parts of it, which are:

  • There’s two girls who decide to rob another rich girl
  • They go to her house and then all three of them play pretend(?)
  • Then one of the two friends doesn’t want to do that anymore and tries to run away but the other two find her and drug her to keep her there. In order to make sure she remembers it later, she writes it in her diary. One day she finds this diary and realises what’s going on.
  • There’s also a cop killed by them at some point.