Given the conditions you described (ie, no oncoming traffic), surely he would have had full beam on? With no blind corners I would have thought that would have given him much more time to react. Of course, having the car overloaded would have caused the car to take a lot longer to brake. Anyway, as said before, if the insurance company hears about the 7 people, they will probably disown him; any excuse to get out of paying up.
Moot point, but is it actually legal requirement for a warning triangle to be carried, or only a suggestion? A triangle 100 metres away would certainly give anyone oncoming a better chance to stop, assuming of course there was time to set it up.