First of all, a tenant's intent to vacate should always be in writing. You in reply, should confirm in writing that tenants intent that way it would be considered a covenant. You stated that you have someone else interested in the unit. Has that new prospect given you a deposit on the rental unit and a signed agreement? Or do you need to confirm that the prospect is still interested. If in fact they are interested and have contracted the unit, you tell the vacating tenant "sorry" it's already rented. Everything should be in writing.
Thank you, and I did send her a certified and return receipt letter stating her intentions, and that I needed it in written she has not done this at this point. But she did send me text message on my phone (will that work?)