。
She was envious of her neighbor's good fortune.
Literal
She [topic-は] neighbor [possessive-の] person [possessive-の] good-fortune [object-を] was-envying.
妬む (ねたむ, 'to envy, to be jealous of') takes を for the thing envied. 隣の人 ('the person next door / neighbor') is a natural way to refer to a neighbor. 幸運 (こううん) is a Sino-Japanese compound for 'good fortune.' The ~ていた marks a past ongoing state of envy.