。
She must have worked very diligently.
Literal
She [topic-は] very eagerly worked [must have-にちがいない].
Pairing past-tense 働いた with にちがいない gives 'must have worked' — the inference applies retrospectively. The whole clause 熱心に働いた is wrapped by the certainty marker. にちがいない sits at the strong end of the inference scale; weaker possibilities would use ~かもしれない or ~だろう.