。
She was just about to head out.
Literal
She [topic-は] precisely going-out [about to-ようとしていた].
The pattern ~ようとする ('be about to / try to') captures the moment just before an action begins — formed from the volitional ~(よ)う (the speaker's intent or willingness) plus とする ('to try, treat as'). In the past progressive ~ようとしていた the imminence is frozen at a past moment: 'she was on the verge of going out.' まさに sharpens this to 'right at that exact instant.' 外出する is a stiffer, more written-flavored alternative to 出かける.