Maybe we just had an "off" day?
The most helpful and encouraging thing to say? Is probably not, "Welcome to the rest of your life."
Even if it's true, you know?
And if you have directed her to write a document which is exhaustive in a way that other students' versions have never been, she's been telling you the precise length of said document for months as it surpasses successive decade marks, you've been nodding saturninely at those reports while making remarks to the effect that she needed to "really make some progress," when she then presents you with a mostly finished draft of said document . . . .
Looking shocked at its heft and telling her that her committee is going to balk at reading said document and question its practical utility? Also not so helpful.