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I'm not buying "NDA" as a reason for his silence. I'm pretty sure you can't use an NDA like that.

If the company itself claims - or at least, refuses to correct the impression by others - that he's still with them, then if he is still working for them he can say so (because he's only confirming what they've already intimated), and if he isn't he can also say so (because they're defaming him otherwise).