It's unfortunate that some people do not have any integrity. Tell her that if she doesn't pay her rent (as agreed to in the lease) you will report her to a national "dead beat tenant" list and this will make it impossible for her to get another apartment.
State the you would rather end the relationship on a good note, but if she decides to not pay her rent, then you have no other choice.
Also, tell her you would rather give her the security deposit back, but you will only do so if all her rent is paid up and the apartment is returned in good condition.
If she refuses to cooperate, tell her you have no choice but to report her to the service and the credit bureau (you actually may or may not want to do this).
If she doesn't pay, then keep the amount of security deposit you need to cover the missing rent AND to bring the apartment back into the right condition. Ensure you take pictures of any damages and list out everything you are decuting from the security deposit. You may need this for backup.