While being able to fly the plane from the right seat is necessary to this exercise, it is not sufficient. It is also necessary to be able to evaluate the performance of the pilot flying, determine when intervention is necessary, and know how to intervene, take control, and recover from whatever situation into which the pilot flying has put you. Those are the hard parts of the flying side being an CFI, and in the case of acting as PIC from the right seat while someone else flies the plane, those skills and knowledge are essential for safety. While it's possible to acquire them without being a CFI, few light GA pilots do. Hence, my statement you quoted -- the CFI training he's completed may have given him those skills, or it may not, and he'll have to decide for himself whether he's capable of fulfilling that role.