。
She isn't so naive as to say something like that.
Literal
She [topic-は] that-kind-of thing [object-を] say [to-the-extent-of-ほど] naive-about-the-world is-not.
世間知らず (seken-shirazu) is a great compound: 世間 ('the world, society') + 知らず (the classical negative continuative 'not knowing'), giving 'one who doesn't know the world.' Used to describe naive, sheltered, or socially inexperienced people. The judgment is gentle in some uses (a kid leaving home for the first time) and pointed in others (a clueless adult who should know better). The sentence uses ~ほど ('to the extent of') to set a threshold — and then denies that she meets it: 'she isn't so naive that she would say such a thing.' Implies the speaker has confidence in her social savvy.