。
She must have been rich.
Literal
She [topic-は] rich was-(noun-predicate-past) must-be.
~に違いない pairs with a past-tense predicate to express past-tense inference: 'must have been X.' Note that the noun-predicate past だった precedes に違いない without any adjustment — Japanese keeps the inferred state's tense in its own marking, while the inference itself stays untensed. The construction is one of the strongest in Japanese for expressing inference: stronger than ~だろう ('probably') and more confident than ~と思う ('I think').