He needs an indisputable paper trail showing that the money was his. That’s a lot of cash and there’s little reason to keep all of your legitimate fiat savings in cash like that.
I’m not sure of how to read your post, but you seem to be implying that the burden is on him to prove that the money is his, when he wasn’t doing anything illegal by carrying it, and nobody has alleged otherwise.
He already said it wasn’t his and that he didn’t know how it was hidden in his car. Whether under duress or not, the authorities aren’t giving the money back as a result.
