If he has a good working cooling system with a 13lb cap it will not boil until it reaches over 109c.

As suggested I would run an 80-82 thermostat for starters, but some of your radiator tubes could be partially blocked. I have found that if 10% of the tubes are blocked on a hot day they generally start to run a bit hotter than normal. I wouldn't have a car without a good fan and shroud. Although what part of NZ are you from , you probably wouldn't even need a radiator if your deep down south.