If at all possible, do not use Aerostart or any other ether spray magic potion. These substances knock seven bells out of piston rings and valves.
It is VITAL he works out FOR SURE that his glow plugs are NOT wired in series, and as it is a Turbo 2.5 they should NOT be. There should be one hefty cable joining the tops of the 4 plugs, and one cable coming from the dash somewhere.
10 to 11 volts is about right.
Here's two good tricks to get one of these running:
1. TOW START IT ! Starter motor speed is very slow on a cold day, probably 60 rpm or less, and 100 rpm when warm. Not enough to fire up, a chilly diesel. Towing it at 15 mph and second gear will give about 1,500 rpm, and she'll fire up damn quickly for you.
2. This needs two people. Take off the glow plug feed cable, and get a second well-charged 12 volt battery (even quite a little one will do) and jump leads. Connect the black jump-lead from slave battery minus (- ) to the chassis, and connect the red jump lead to the slave battery positive ( + ). Keep the left-over red jumper lead in your hand, and well clear of the bodywork. Get number two person in the driver's seat, check it is in Neutral, with the clutch down to reduce gearbox drag. Now you hold your left-over red lead onto any glowplug tip and count up to 20 seconds at least, and still with the jumper still touching on the plug-top shout at number two to give her a whirl till she's running, at which stage you can disconnect your slave leads.
Diesel start technique should NEVER be short bursts on the starter - keep it turning for a good long time, and it should speed up as it starts to fire up a bit, and eventually it should catch.
As mentioned, these are difficult engines to start from cold - all indirect diesels are. So, they need a really good glow to fire them up, and this slave battery trick is one way to keep the plugs FULL HEAT on 12 volts even during starter motor operation when the voltage might drop as low as 5 to 8 volts.
CharlesY