。
She got angry at his bad manners.
Literal
She [topic-は] his [genitive-の] ill-mannered [adj-な] behavior [at-に] belly [object-を] raised.
腹を立てる is a set phrase for 'get angry, be irritated' — literally 'to raise one's belly.' Japanese has a constellation of anger idioms anchored in the 腹 ('belly, gut') metaphor: 腹が立つ (intransitive partner — 'belly stands up' = get angry), 腹に据えかねる (couldn't keep it inside), 腹を割って話す (talk frankly, 'split the belly'). The に on 振る舞い marks the cause/source of the anger. 振る舞い is the noun form of 振る舞う ('behave, conduct oneself').