。
There's no way she could have known about it.
Literal
She [topic-は] that [about-について] [knew-知っていた] [no-possibility-はずがない].
~はずがない is the strong-denial frame — 'there's no way (it could be that) X / it can't possibly be the case that X.' Built on the noun はず ('expectation, supposed-to-be') + が + ない ('there isn't'). It denies the possibility of the embedded claim. Pair with ~はずだ ('it should be the case that'), the affirmative counterpart. Here 知っていたはずがない denies that she had been in the state of knowing — so the speaker is asserting that her ignorance must have been genuine. Common in argumentative, defensive, or detective-fiction contexts.