This is my best understanding...

The head gasket went as a result of the radiator leaking. The coolant was emptied from the engine the thermostat wasn't recording a high engine temp as there was no fluid to read the temperature from. Hence the head warped and pistons were likely softened.

The car had been driven for ~10 miles before losing drive and the power steering went, as soon as this happened the driver read the engine temperature as max and the car couldn't be restarted by theAA and had to be towed.