。
She's insisting that her son is innocent.
Literal
She [topic-は] son [subject-が] innocent is fact [object-を] is-asserting.
Three patterns. 無罪 ('innocent, not guilty') is the legal term, opposed to 有罪 ('guilty'). [clause]+であることを is the formal nominalization frame — であること packages a copula clause as a noun. Compared to ~ということを, であること is more literary and asserts the fact more directly. 主張する ('to assert, insist on, advocate for') is a strong claiming verb, often used in legal, political, or argumentative contexts. The sentence carries a courtroom feel.