。
She isn't fool enough to talk back to him.
Literal
She [topic-は] him [to-に] talk-back [extent-ほど] foolish is-not.
An instance of the ~ほど~ではない ('not enough to, not so much that') pattern, with 口答えする ('talk back, retort') as the threshold action. The に on 彼 marks the recipient of the imagined retort. 口答え is the 連用形 + する compound; the noun 口答え + する converts cleanly to verb. 愚かではない is the polite/written negation of 愚か ('foolish, silly').