EXACTLY!!! Now you are thinking like me! 
Oh and by the way, when it comes to carelessly leaving out something valuable - Make sure that it in really has no value. I know that many jewelers make paste jewelry (fake jewelry that is), which they use to put on display. So ordering some of that will do the trick! I am sure the stupid 19 year old will not be able to tell real from fake…
Anyway once she has been caught, then of course she should be made to get a job - So she can pay back her parents and elder sister!
