。
She believes in his innocence.
Literal
She [topic-は] his [genitive-の] innocence [object-を] is-believing.
信じる ('to believe') takes the believed thing with を — note the contrast with English 'believe in,' where Japanese doesn't add a preposition. The ~ている here marks ongoing belief (a held conviction), not a momentary act of believing. 無実 ('innocence') is specifically the legal/moral sense — innocent of a crime or accusation — distinct from the more general 純真 ('pure-hearted, naive').