To be honest, unless he's planning on doing 20k plus miles a year i'd avoid the higher purchase price, higer servicing costs, and higher likely costs of repair of a biggish diesel saloon

Petrol is cheaper and will remain so, petrol cars are cheaper to buy, petrol cars are more reliable IMO with less likelyhood of meeting big bill

The money saved on the purchase and servicing and the lower cost of petrol will even out the ownership cost despite the lower mpg if you get a sensible petrol model unless he's doing biggish mileages