Did she "threaten to resign"? "Threatening to resign" implies that you believe you are so critical to the company that the company will suffer in your absence.

There is nothing in the email to suggest that. Disagreeing with company policy doesn't mean that she was not a diligent worker doing the tasks she had been assigned to in good faith.

So far Google hasn't stated the thing she did exactly that led her to being fired, ie the actionable thing she did that would have to be produced if she took it to a tribunal or to court?