Does she really need a diesel? Does she have a commute of over 15 miles each way or travel around the country most days? If not, stick with petrol.

The 1.6D pulls well, is fairly smooth and can return good economy figures.

The 1.6 petrol is rather gutless in the V50/S40. The 1.8 is a better bet and a smooth performer. Economy is similar to the 1.6.

The only real way to decide is to try some out. You might need to pretend to be looking for a C30 or S40 to try out various engines if you can't find enough V50s locally. They all have the same engine availability and are identical up to the B pillars.