。
She must have been rich at the time.
Literal
She [topic-は] that-time rich was [must be-に違いない].
~に違いない ('there's no doubt that / must be') expresses strong certainty — stronger than ~だろう or ~はずだ. Attached to the past form 金持ちだった, it becomes a confident inference about the past. 当時 ('at that time') anchors the statement historically. 金持ち is a direct, informal word for 'rich person.'