If they were saving money, they'd buy the most fuel-efficient, cheapest car they could. Or not buy a car at all.
People do not buy cars to save money. It is not the act of a rational person to suddenly come over all frugal after deciding to buy a car and then subjectively picking one car for arbitrary reasons.
Once more - if you're looking to save money, you do not buy a car.
I must add something that I think you are missing from your first statement, which I think hurts what you are trying to say a bit.
If they were saving money, they'd buy the most fuel-efficient, cheapest car they could. - that fits their needs.
Need room for more than two toddlers, and want to enjoy driving? But you have a specific budget to work with in both car cost and fuel efficiency?
If car decisions were purely about saving money, then yeah, we would all still be driving around in Chevettes. But even for those who are looking at it as just a tool, then it does require more thought than that. How many people will need to fit in the car? How safe should it be for you to feel comfortable? How reliable is it? A cheap car is pointless if it gets repairs every other week.
You made some kind of leap, or at least appear to, that buying a diesel due to fuel efficiency would mean that is priority 1. But if they settled on a car design and are choosing between a gasoline or diesely version then obviously they had a list of criteria first and now the efficiency question is coming up.
I knwo when I looked for a car I knew my affordable budget, and what features I wanted it to have. Then after I found cars that fit those criteria I then looked at efficiency and said, "Can I afford it?" Efficiency is secondary for many people. The people who use efficiency as a primary criteria immediately go look at hybrids. Then very few of them get beyond that point.
No, if someone is looking at a mid-size or larger car and trying to decide on the specific model based on efficiency then they had other things in mind first, and likely tiny cars or buses will not meet their needs/desires.
In this very specific case efficiency is one of a list of things that must be met. He has the ability to afford something around $30k. He would like it to have all kinds of specs, efficiency being one of them. If efficiency were his primary goal then he would have eliminated half his other criteria because a Yaris would fit the efficiency needs. So, diesels were suggested in order to help him find a car that will fit his criteria. I have no doubt that the final decision will still come down to a test drive, as it does with most people.