。
She regarded him as an idiot.
Literal
She [topic-は] him [object-を] dimwitted person [as-と] regarded.
みなす ('regard, consider, deem') is the most decisive of the 'consider X as Y' verbs in Japanese — stronger than 思う (think) or 考える (consider), it implies a judgement that's been settled and is now treated as fact. 間抜け ('dimwit, fool') is a colloquial insult, na-adjective; 間抜けな人間 = 'a fool of a person.' The Xを Yとみなす pattern is parallel to Xを Yと思う but with the implication 'I've decided X is Y, that's how I treat them.'