。
She had been there ten years before that.
Literal
She [topic-は] that 10-years before there [to-に] had-been-going.
そのＸ年前 ('X years before that') uses そ- (the medial demonstrative, referring to a previously established time) to anchor the calculation — 'ten years prior to that earlier moment'. 行っていた is the past durative of 行く ('go') — but here it functions as a perfect-aspect 'had gone, had been there', describing a state established before the reference time. This is a subtle but real use of ~ていた: not just 'was going' as a process but 'has-been-there' as a settled prior state. Japanese telescopes English perfect tenses ('had been', 'has been') into the same ~ている form, with context picking the meaning.