。
She used to be diligent.
Literal
She [topic-は] long-ago diligent was.
勤勉 (きんべん) means 'diligent' or 'hardworking' — a na-adjective. 昔 (long ago) combined with the past tense だった creates a simple before-and-after contrast: she was diligent in the past but the implication is she no longer is. This is a minimal, unadorned statement — the listener fills in the rest.