。
She was in that play.
Literal
She [topic-は] that play [in-に] was-appearing.
~に出る is a multi-purpose 'appear in / come out / go out' verb that pairs with arenas of activity — 劇 ('play'), 試合 ('match'), 番組 ('TV show'), 大会 ('tournament'). The に here marks not motion toward but inclusion within: she was 'in' the play, not heading 'to' it. ~ていた is past durative, fitting for a stage role spanning a run of performances: not a single appearance but an ongoing 'being in it' over time. Note that 出る in this performative sense is the inverse of its everyday meaning of 'go out / leave' — a participation in something is, from the audience's standpoint, a coming-out *onto* the stage.