。
She heaved a sigh of relief when she finished her work.
Literal
She [topic-は] work [object-を] finished [time-とき], with-relief sigh [subject-が] came-out.
A sentence packed with sensory mimetics. ほっと ('with relief, with a feeling of release') is a 擬態語 (state mimetic) for the feeling of tension lifting — often paired with する (ほっとする 'feel relieved') or with sigh-related verbs. ためいき (also written 溜息) is 'a sigh.' ためいきが出る ('a sigh emerges') uses the intransitive 出る for spontaneous physical reactions — Japanese frames many bodily responses as things that 'come out' rather than things one actively does: あくびが出る ('yawn comes out'), 涙が出る ('tears come out'), 熱が出る ('a fever comes on'). The とき clause sets the time of the sigh.