The CDR signs for the jet and is responsible for it and all the occupants until everyone is off and in the terminal. Therefore, with 2 SOs and a knackered RQFO, he should be doing the landing. If he assesses that the FO is not fit for purpose (too tired) he should declare an emergency as the 2 SOs are not licensed below 20K ft and, therefore, technically the flight is now single pilot. A minimum of an MOR and an ASRF would be required, in addition to the investigation under ICAO ANNEX 13. How long before the second FO would be reinstated?