。
She must have done it yesterday.
Literal
She [topic-は] yesterday that [object-を] did [must be-に違いない].
~に違いない is the strong-conviction conjectural pattern — 'must be, there's no doubt that.' Stronger than ~だろう ('probably') and ~かもしれない ('might'); it expresses the speaker's near-certainty based on evidence or inference. Attaches to nouns, plain-form verbs, and adjectives (with の or directly). Here したに違いない reads as confident deduction: the speaker has reasoned their way to this conclusion. Common in detective-novel dialogue, but equally at home in everyday inference about absent third parties.