。
She's a hard worker.
Literal
[topic-は] hard works.
よく働く ('works hard') uses よく ('well') as an intensifying/qualifying adverb on 働く ('to work'). Note: in this very short sentence with no past-tense or single-event marker, よく leans toward 'often, frequently' — but with 働く it conventionally means 'works hard' (well at working). The dictionary form 働く describes a habitual or characteristic property of the subject, much like English present tense.