Not straightforward. He'll be looking at a difference in economy of about 20%. But ordinary diesel costs more than petrol (the same as superunleaded), the car will cost more for being diesel, and if it's done 60k miles already, over the next 75k miles taking it to 135k miles he could need a DPF, a DMF, an EGR, a turbo, whereas the old chain cam 2.0 litre petrol engine in the Mondeo is fairly bulletproof.