Buy a petrol and save some money.Mondeo 1.8

The Mondeo would be a good choice, cheap, well equiped and reliable. If he is carrying some serious weight the Citroën C5 is well worth a look thanks to the self levelling suspension and is cheaper still, you get more niggly (usually electrical) problems with the C5 but they are reliable in the sense that they don't break down very often. Both cars share the same diesel engines in slightly different tunes.

If you can find one, older generation Mazda 6 estates are bullet proof.

As the other posters said, petrol will be more reliable. if you get a good diesel for £2K great - but it is rather a gamble.

Left-field pick: A well looked after late (2000) Citroën XM 2.1 TDi estate would be a good buy, being an "old tech" diesel they are reliable, XMs are supremely comfortable on the motorway, probably the most comfortable car this side of a Roller. Mk1 XMs were terrible, late ones are pretty reliable.

For £2k he should choose on the basis of condition rather than make or model, although starting the search with a Mondeo or Octavia would make sense.

I agree with the others - he should go for petrol unless he does a huge mileage, which for his sake I hope he doesn't if he's obliged to buy a high-miler in the first place.