I too found it astonishing to read about the DA40. How can somebody buy a DA40TDI, approx £180k+VAT and easily £200k+VAT with a half-decent IFR fit according to the Diamond stand at the Earls Court show today, and not know it doesn't take avgas?
On the other hand, look at how many people there are in GA who will tell you something absolutely categorically, and it's pure b******t. Some of these, as I know only too well to my cost, are instructors, and not always in ignorance.
Obviously this is speculating, but this man could have been a relatively new PPL and someone who sounded like they knew everything about aviation (ever met the type??) told him it's OK. Perhaps the someone thought the engine was a turbine? How many PPL instructors know what a DA40 is?
Lots of possible explanations.
I was once told by a very qualified ME/IR instructor that I could buy a 1/9 share in a Private CofA plane and do an IMC Rating in it. I would have been a right idiot to join that group (of which he was a member). Anything goes in this business, unfortunately.