。
She must have been doing well financially back then.
Literal
She [topic-は] long-ago spending-power [subject-が] was-good [must be-に違いない].
羽振りが良い (はぶりがよい) is an idiom meaning 'to be doing well financially' or 'to be riding high' — literally 'wing-flapping is good,' an image of a bird spreading its wings freely. ~に違いない expresses strong conviction based on inference: 'must be' or 'no doubt.' Stronger than ~だろう, this pattern leaves no room for uncertainty.