。
She fell head over heels for him at first sight.
Literal
She [topic-は] at-one-look him [in-に] fell-deeply-in-love.
惚れ込む ('to fall deeply in love / become smitten with') is a compound verb: 惚れる ('to fall for') + 込む (productive aspectual 'deeply / thoroughly'). The ~込む suffix turns the action into something more intense, more committed — compare 思い込む ('be convinced of'), 信じ込む ('believe deeply'). 一目で ('at first sight') sets the speed; the に on 彼 marks the target of the falling. 一目惚れ ('love at first sight,' a noun) is a popular phrase in Japanese romantic culture and titles of dramas, songs, manga.