There is nothing wrong with Ben's camera. The flash was set to commander mode, with '--' for the built-in flash mode. No return light was detected from the flash because it only fired the triggering pulse which occurs too early to be counted, and no external flashes were being commanded. There was no response to the metering preflash, either, so the camera's meter did not know how much light any external flashes might add to the exposure. Like so much of our camera systems, the behaviour is undocumented, but it seems the camera just uses the user-set value for ISO when it cannot make a good guess at what the exposure should be.
Some of this may be conjecture, but I have repeated the behaviour I described above on my D300s. It also explains why Ben's ISO 100 picture was grossly underexposed when we'd expect a reasonable exposure at ISO 100 and f/1.8 with the built-in flash.
It seems I was wrong about the Hi1 value not being accepted by Auto ISO, and that Hi1 is allowed and therefore my earlier question mark over that has diappeared.
I repeat: There is nothing wrong with Ben's camera, though the Built-In Flash settings may need to be adjusted.