What do you want him to do to prove it? Buy a 30 year old one and a new one and cut them in half? Send in metal samples to a material science lab?

It’s well-established that the vintage stuff is high quality, and the people who buy the new stuff seem to have similar opinions. I don’t see any reason to believe the modern Le Creuset Dutch ovens are subpar.

If anyone needs to provide evidence, it should be you. Our null hypothesis should be that the quality remained the same. If you want to claim the quality went to shit, the burden of proof is on you