。
She couldn't stand his ill-mannered behavior.
Literal
She [topic-は] his [genitive-の] ill-mannered [adj-な] behavior [subject-が] couldn't-endure.
Notice the case-marker shift: with potential verbs (我慢できる), the formerly-direct object often takes が rather than を — 振る舞いが我慢できなかった rather than 振る舞いを我慢できなかった. This が-marking with potentials is a hallmark Japanese pattern (お酒が飲める, 漢字が書ける) — the 'desired/possible' object becomes a kind of subject. ふるまい is the same word as the kanji 振る舞い written in kana — a stylistic flicker.