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To be honest, she's completely untrustworthy.
Literal
She [topic-は] truth-of-the-matter completely trusted cannot-be.
実のところ (in truth / to be honest) introduces a frank assessment. 全く (completely) intensifies the negative: she's utterly untrustworthy. 信用できない (can't be trusted) uses the potential negative of 信用する (to trust). The sentence is a blunt, private judgment — the kind of thing said behind someone's back.