if long runs of self sustaining were possible then why not just unplug and produce power for a month or two.
That should "prove" things and get him all the attention and offers he could ever handle.
If you have a self sustaining system with even a few watts of heat then you should be able to generate electricity
with no problem (Peltiers, Stirling engines.....)
Such a demonstration would be very appealing and you would not need fake customers, or steam quality assumptions, and so on.
My guess is that he does not have such a system or he is a fool not to try such a demo.