OP-
You keep saying your deposit will cover the last month's rent. Is it stated in the lease that the Security Deposit is to be used for the last month's rent?
If it isn't, you better pay the last month out of your own pocket and then have her return the security deposit to you. The security deposit is used for more than just the last month's rent. It is supposed to be used for any damages and what-not...and can be used in the event that the tenant failed to pay rent, which sounds like you're trying to do.
You keep saying your deposit will cover the last month's rent. Is it stated in the lease that the Security Deposit is to be used for the last month's rent?
If it isn't, you better pay the last month out of your own pocket and then have her return the security deposit to you. The security deposit is used for more than just the last month's rent. It is supposed to be used for any damages and what-not...and can be used in the event that the tenant failed to pay rent, which sounds like you're trying to do.