。
She has very good manners.
Literal
She [topic-は] very manners [subject-が] good.
行儀がよい literally 'behaviour-is-good' is a fixed expression for 'well-mannered, well-behaved.' 行儀 itself bundles posture, table manners, and general comportment in social contexts — it's the kind of thing parents drum into children. The double-subject frame (彼女は + 行儀が) is the canonical 'X has Y' shape: large topic, small attribute.