。
She must have been quite a beauty when she was young.
Literal
She [topic-は] young days considerably beauty was [must be-にちがいない].
ずいぶん (quite, considerably) adds emphasis beyond とても — it implies the speaker is genuinely impressed by the degree of beauty they're inferring. 美人だった uses the noun 美人 rather than the adjective 美しい, framing her as 'a beauty' rather than 'beautiful.' The combination of ずいぶん with に違いない expresses a confident and emphatic inference.