、。
She complained to me about his rudeness.
Literal
She [topic-は] his [genitive-の] rudeness [about-について], me [to-に] dissatisfaction [object-を] said.
~について is the standard 'about, concerning' compound particle — used for the topic or subject matter of an action or discussion. The に here marks the recipient of the complaint (me, the listener). 不満を言う ('say a complaint, complain') is the verb-phrase counterpart to 不満を持つ ('hold a complaint, be dissatisfied'). Together the sentence specifies all three roles: complainer (彼女), listener (私), complaint subject (彼の無礼).