。
I fell in love with her the instant I met her.
Literal
I [topic-は] met [instant-瞬間] [at-に] her [object-を] liked-became-[completion-てしまった].
瞬間に ('at the instant') marks the moment with an explicit に. ~てしまう conveys that the falling in love happened involuntarily and completely — it was beyond the speaker's control. 好きになる ('come to like') with the completive auxiliary suggests a sudden, total tumble.