。
She got the wrong impression of him.
Literal
She [topic-は] him [about-に] mistaken [past-った] impression [object-を] held.
抱く (here read だく, also possible いだく for the figurative use) literally means 'embrace, hold,' but extends to abstract things one 'holds' mentally — feelings, impressions, opinions, dreams, doubts. 印象を抱く ('hold an impression') is a common collocation, more formal than 印象を持つ. The に on 彼 marks the target of the impression. 間違った is the past form of 間違う used as a prenominal modifier.