I'm not a lawyer but even if it was, eg. a year's salary at the time she accepted it, is that really a fair price for a lifetime of silence?

That would be up to her, wouldn’t it?

And she signed it, so presumably it was for her.

Maybe? Is your argument that there is no fair price, or that it wasn’t enough? The former makes NDAs unenforceable.

NDAs cannot cover whistleblowing of actual criminality, including sexual harassment, which is why modern NDAs take pains to disclaim that, so they wouldn't be invalidated on that basis. Presumably the behavior exposed in the book, while arguably immoral or amoral, doesn't rise to the standard of criminality.

Writing a book isn't covered whistleblowing. If she wants to go to the FBI or whatever, no one can stop her.

Exactly. She was not prevented from doing that.

Exactly. Or a legislature as Frances Haugen did.

That said, arbitrators like that which gagged her are inherently conflicted since they are paid by the corporation, sadly our corrupt Supreme Court rubber-stamped binding arbitration so she has no recourse.