。
She couldn't put up with his rudeness any longer.
Literal
She [topic-は] his [genitive-の] rudeness [at-に] this [more-以上] couldn't-endure.
我慢する ('to endure, put up with, hold out') in its potential form 我慢できる ('can endure') makes the sentence pivot on the ability — '(she) couldn't endure.' The negative past 我慢できなかった is the verdict that her tolerance ran out. これ以上 ('any more, any further than this') sets the threshold. The に on 無礼 marks the source/object of the enduring — 'endure (against) rudeness.'