My question involves landlord-tenant law in the State of: Florida
Helping a family member who is renting a single family house to two tenants in Florida. At the beginning of the lease they paid a full first and last month's rent as well as a security deposit, and the lease began on the 12th of the month and not on the first, with a due date on the first of the month and with the lease ending on the 12th. One tenant is leaving at the end of the lease, the other is renewing/signing a new one year lease, with a new roommate at the same property. The money paid at the beginning of the original lease was paid by the tenant who is staying in the property by cash, and it's unclear as to how much each of them paid toward the money that was due.
My question is: how do I handle the first and last month's rent on this, as it was not prorated and not specified in the lease how it was to be handled? Would I extend the lease to the end of the month (the departing tenant has already moved their belongings out but have not given formal notice) and credit the tenant who is staying, or is there some other action I should be taking?

