。
She had been very surprised at the sight.
Literal
She [topic-は] that scene [target-に] greatly was-surprised.
たいへん ('very, greatly, enormously') is an intensifier — heavier and slightly more formal than とても. The progressive ~ていた frames the surprise as ongoing or a maintained state at a past reference point — not just the moment of being startled, but the lingering surprise that endured. に marks the trigger of the emotion. The combination gives a narrative flavor: 'at that point, she was [still] in a state of being very surprised by the sight.'