Sounds to me like your friend has been receiving lots of excuses and not much actual repairwork.

There isn't much to stop a diesel engine from running. You need fuel, air, compression, and heat. Thats about it.

If theres a leak in the fuel delivery, it won't get fuel. You can tell by looking at the clear fuel pipes near the filter, if there are air bubbles in them while the engine is running, you have a leak. If theres fuel dripping from the underside of the shutoff valve, you have a leak. They're all easy enough to service, its DIY stuff, you don't need any special tools, perhaps a battery charger for when your battery runs out from all the cranking. The fuel system works under vacuum, so you won't see diesel spurting out anywhere. It'll just suck air in. Note that its normal to see a small bubble of air in the pipe that runs to the pre-filter when the engine is stopped. A small bubble that is, not a humungous great whacking 6 inches of air

The other suspect is obviously the glow plugs. They make it easier for the fuel to combust. If they're not working, you'll have a very hard time starting it, particularly in cold weather. The glow plug relay is mounted on the driver's side, near the top of the wheel arch. Small black box. Remove it, open it up, you'll see 6 large fuses - check they're intact. If the relay is working (check its getting 12V btw), then check the glow plugs. You can get to the first one, but the other 5 you'll need to remove the intake manifold - unless you test them from the glow plug relay.

Honestly, once you have a basic idea of how these engines run, they're pretty easy to fault find. Theres not much to go wrong.