。
She had been hating him.
Literal
She [topic-は] him [object-を] was-hating.
嫌う is the verb form 'to dislike, hate' — distinct from the adjectival 嫌い ('hated, disliked'). The past continuous ~ていた ('was/had been doing') marks the dislike as an ongoing state in the past, often setting up an implied 'but now…' or 'until something changed.' Notice that 嫌う takes the object directly with を, unlike 嫌い which takes が (for the disliked thing) — a common point of confusion for learners.