。
She would never tell a lie.
Literal
She [topic-は] never lie [object-を] does-not-tell [probably-でしょう].
うそをつく is the standard idiom for 'to tell a lie' — the verb つく here is 付く ('attach'), not 'arrive,' so it parses as 'attach a lie [to one's words].' The 決して~ない pattern adds emphatic finality, and the trailing でしょう ('probably / I expect') hedges this from a flat assertion into a confident inference: 'I'd bet she'd never lie.' The tone shifts from declaration to character endorsement.