。
She heaved a sigh of relief when work was over.
Literal
She [topic-は] work [subject-が] finished [time-とき], with-relief sigh [object-を] applied.
A pleasant little release: ためいきをつく ('let out a sigh, to sigh') is a fixed collocation where つく ('attach, put') pairs with breaths and sighs across many idioms (息をつく 'catch one's breath'). ほっと ('with relief') is a 擬態語 (state mimetic) for the feeling of tension lifting. The とき clause sets the time of the sigh — when the work was finished. Compare ためいきが出る ('a sigh comes out'), nearly synonymous but framed passively, as if the sigh emerges by itself rather than being actively produced.