。
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] [should not be (existing)-はずがいない].
Standard はずがない ('there's no way') swaps in the verb いる ('exist for animate beings') in place of ある ('exist for inanimates'). The result はずがいない is non-standard — formal grammar prefers はずがない regardless of animacy, since はず is an abstract noun rather than a living thing. The form here may reflect a colloquial slip or an older variant; you'll occasionally see it in older texts and casual writing, but it's not the canonical form a learner should imitate. Treat はずがない as the safe default.