。
She had matured into a wonderful woman.
Literal
She [topic-は] lovely woman [into-に] had-matured.
成熟する (せいじゅくする) means 'to mature' or 'to ripen,' used here for personal development. The に marks the result of the transformation — what she matured into. The ~ていた form captures a completed state observed in the past, giving a reflective, narrative quality: the speaker is looking back at a transformation that had already taken place.