He said of course it can. I said prove it.
I doesn't work that way. If the NEC doesn't say you can't do something, then you can do it. It is your responsibility to prove that he can't do it.
Looks like we're back to green or bare for the GEC??
No. I'm in the camp that consideres a GEC to a grounding conductor, not a grounded conductor. But, it is not an equipment grounding conductor. Therefore, there is no code section that requires it to be any particular color. It can not be white according to section 200.7. It can not be a color that would cause confusion with an ungrounded conductor per 310.12.
Try this one. Those of you who would consider the GEC to be a grounded conductor can not install a grounded electrical system. According to definition, a grounded conductor is a system or circuit conductor. The GEC is not a circuit conductor, therefore it must be a "grounded system conductor". 200.6 requires it to be marked with white or gray since it is a grounded conductor. 200.7 prohibits it from being marked by any of those means since it is not a "grounded circuit conductor". Therefore, it can not be installed