。
She is convinced he is innocent.
Literal
She [topic-は] he [subject-が] not-guilty [quotative-と] is-convinced.
思い込む is a compound verb: 思う ('think') + 込む ('go deeply into'), giving 'to be set in one's thinking' — i.e., to be convinced or to convince oneself, often with a slight nuance of being mistakenly persuaded. The ~ている marks an ongoing state. 無罪 is the legal 'not guilty' — distinct from 無実 ('factually innocent').