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She pretended she hadn't heard him.
Literal
She [topic-は] him [agent-に] not-having-heard pretense [object-を] did.
聞こえる ('be audible') is intransitive — the thing heard is the subject, and 彼に here marks the source ('from him'), so 彼に聞こえなかった means 'his voice wasn't audible (to me/her).' Wrapping that whole past-negative clause in ~ふりをする ('to put on a pretense of...') gives 'pretended she hadn't heard him.' A common everyday strategy for ducking a conversation without overtly refusing it.