。
Believing he was innocent, she helped him.
Literal
She [topic-は] he [subject-が] innocent [quotative-と] believe-and he [object-を] helped.
A quotative-と + 信じる pattern: the proposition 彼が無実だ ('he is innocent') is the content of the belief. The te-form 信じて links cause/circumstance to the main predicate 助けた ('helped') — 'believing X, she helped.' Two clauses, but Japanese uses a single sentence via te-linking.