To test the thermocouple, you need a 3/8 inch open end wrench and a volt meter that tests millivolts. A thermocouple should test in at 30 millivolts which this one did. The one thing that I did find was that it was very loose inside the gas control, which would cause the problem that the customer (who was at his wits end) described.
I tightened the thermocouple into the gas control snugly with the wrench and lit the water heater. So far, so good. If this is happening to you new G.E. water heater, give this a try. Maranatha!