。
She believes everything he says.
Literal
She [topic-は] anything he [possessive-の] saying-thing [object-を] believes.
彼の言うこと literally 'the things he says' — note the の here marks the subject of the embedded relative clause, an alternative to が that's common in noun-modifying clauses. Without further context, this could read as devotion or as warning: Japanese 信じる carries the same dual valence English 'believes' does — admirable trust or naive credulity, depending on what comes next.