。
She must have finished her work yesterday.
Literal
She [topic-は] yesterday work [object-を] finished-completely [must-be-にちがいない].
~にちがいない ('there is no mistaking that...' — 'must be / surely is') marks strong inference: the speaker is asserting confidence based on circumstantial evidence rather than direct knowledge. Stronger than だろう ('probably') and ~と思う ('I think'); only 絶対 ('definitely') edges past it in spoken certainty. The polite form is ~に違いありません. ~てしまう adds a completion nuance to 終える ('finish'), reinforcing the sense of definitively wrapping up — the work is done and behind her.