。
I was surprised at how late she was.
Literal
She [topic-は] that-much late [because-ので] [I-was-surprised-驚いた].
そんなに ('that much, to that extent') sets the degree of lateness as exceeding expectation, and the ので clause makes the lateness the cause of the speaker's surprise. The sentence is interesting structurally: although 彼女 is grammatically the topic, the surprise belongs to the speaker, who is left implicit. Japanese routinely lets the experiencer of an emotion stay unmentioned when context makes it clear who's reacting — the listener understands it's the speaker who 'was surprised,' not the woman herself. This is one of those sentences where the literal translation loses the natural English subject, which English forces you to insert.