Someone I know has said to me that 'people need a great amount of classical training before they can handle the challenges of jazz harmony, playing and improvisation.'
This sounds like a rather clinical and academic approach to playing jazz - the antithesis, in fact, of the essence of jazz.
I used to listen to a lot of New Orleans jazz, albeit less so these days. I might be wrong, but I think very few of the jazz greats from the heyday of the New Orleans jazz era had any classical training, let alone to an advanced level. As far as I know, they mostly listened to the jazz musicians in the streets, brothels and bars around them, and then emulated them and added their own personality. Some were taught by more advanced jazz musicians, but IIRC, what they were learning was jazz technique (bending notes, for example) rather than classical. I dare say there were exceptions.
Assuming the person who made this claim is a musician, what is his/her background which leads them to this opinion?