。
She opened up to her friends about what was on her mind.
Literal
She [topic-は] her [genitive-の] heart [genitive-の] inside [object-を] friends [to-に] confided.
打ち明ける literally combines 打つ ('to strike') and 開ける ('to open') — 'to break open and reveal,' used metaphorically for laying bare one's inner thoughts or secrets. Notice the structure: 心の中を ('the inside of one's heart') is the direct object marked by を — what gets opened up — while 友人たち ('friends') is the recipient marked by に. The doubled 彼女 again reads as stilted — a native would say 心の中を友人たちに打ち明けた without the second 彼女, since possession is already established by the topic.