。
She quit smoking.
Literal
She [topic-は] quit-smoking did.
禁煙する is one of those compactly Japanese verbs — the noun 禁煙 ('no-smoking') becomes a verb with the light-verb する, meaning specifically 'to quit smoking' as a personal commitment. Distinct from タバコをやめる ('quit cigarettes') which is more colloquial, 禁煙する has the feel of a deliberate, often public commitment — the kind one announces to family or writes on a personal goal list. The Sino-Japanese verb's compactness suits its committed weight.