The issue seems to be does the supplemental green wire ground need to satisfy 250.122 even though the EMT itself already does.
It does occur to me he can shut the inspector down if he installs an IG receptacle on the 120/20a circuit connected to the green wire. (In a plastic box)
Where does a plastic box come into it? They are not permitted in any run of metal raceway unless the box is fitted with a listed means of assuring continuity between its KOs, if fitted with KOs. Given the code language I wouldn't think that a field installed bond could satisfy the prohibition against the use of Nonmetallic boxes for metal raceways.
314.3 Nonmetallic Boxes. Nonmetallic boxes shall be permitted only with open wiring on insulators, concealed knob and-tube wiring, cabled wiring methods with entirely nonmetallic sheaths, flexible cords, and nonmetallic raceways.
Exception No. 1: Where internal bonding means are provided between all entries, nonmetallic boxes shall be permitted to be used with metal raceways or metal-armored cables.
Exception No. 2: Where integral bonding means with a provision for attaching an equipment bonding jumper inside the box are provided between all threaded entries in nonmetallic boxes listed for the purpose, nonmetallic boxes shall be permitted to be used with metal raceways or metal armored cables.
Since Isolated Ground Receptacles do not have the grounding port of the receptacle bonded to the mounting yoke I don't see what a plastic box would achieve either.