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She complained about her boyfriend's pranks.
Literal
She [topic-は] boyfriend's mean-prank [about-に] complaint [object-を] said.
文句を言う (literally 'say complaint') is the standard everyday way to say 'complain.' The に here marks the topic or target of the complaint — 'complain about X.' 悪ふざけ ('mean prank, mischief gone too far') combines 悪 ('bad') + ふざけ (the noun form of ふざける, 'to fool around'). It's not just innocent silliness — the 悪 marks it as crossed-the-line teasing or rude horseplay. ボーイフレンド is a katakana loanword; native 彼 ('he, boyfriend') is more common conversationally for the romantic sense.