Yep, just keep trying. 'Expert' opinions, such as that of a vet, might be taken more seriously if they're so attached to what 'the guy' said. I've met plenty of people like that: they get lucky in that their reptile actually survives and then they think all that crud advice was right and refuse to listen to anyone else.

The 'guy upstairs' was my recent battle, and the fact that I now have that snake is a great ending. The way I did it: I let him come over to see all my snakes and how they were being (properly) kept. Once he saw how a healthy content snake (x9) looked and acted it dawned on him that maybe his methods weren't quite right.

Maybe try something like that yourself? Take them to see some snakes, especially BPs, in proper conditions to see with their own eyes. Get them around a herp vet so they can hear a professional. Sometimes it's the only way to get through.

Good luck!