。
She fell for him.
Literal
She [topic-は] him [to-に] feelings [object-を] directed.
思いを寄せる ('to direct one's feelings toward') is a gentle, literary way to say 'to fall in love with / to have feelings for.' Less dramatic than 惚れる, more poetic than 好きになる. 思い here means 'feelings, affections.' The sentence is simple but the vocabulary choice reveals a literary sensibility.