。
She fell for him at first sight.
Literal
She [topic-は] at-one-look him [subject-が] like became.
好きになる ('come to like') frames the falling-for as a change of state — became someone who likes him. The pattern is the standard XはYが construction used for skill and feeling descriptions: she (X), as for him (Y), came to be in the 'liking' state. Note 彼 takes が (not を or に), even though English would use a direct object — the Japanese frame is 'him is the liked one' rather than 'she likes him' actively. This is the everyday way to describe the spark of attraction; flashier verbs like 惚れ込む carry more dramatic, almost obsessed connotations.