。
She quickly fell for him.
Literal
She [topic-は] soon him [object-を] liked-became.
The standard XはYが好きだ ('X likes Y') pattern flips to を when 好き shifts into the change-of-state 好きになる ('come to like / fall for'). Strictly prescriptive grammar still treats 好き as an adjective and prefers が, but in practice を is widely used with 好きになる because it's reanalyzed as a single transitive verb meaning 'come to like'. Both 彼を好きになる and 彼が好きになる are heard. The acquisition of feeling — the moment of falling for someone — is exactly when this transitive reading feels most natural.