。
She tipped off the police about his hiding place.
Literal
She [topic-は] his [genitive-の] hiding-place [object-を] police [to-に] secretly-reported.
密告する ('to inform secretly, tip off') combines 密 ('secret') + 告 ('inform, report') — implies covert, often anonymous information, typically betraying someone to authorities. Carries strong negative connotations (snitching, informing). 隠れ場所 ('hiding place') is a 連体修飾 phrase — 隠れる ('hide') + 場所 ('place'). The に on 警察 marks the recipient of the information.