。
She's never seen it before.
Literal
She [topic-は] before [at-に] it [object-を] saw [thing-こと] [subject-が] doesn't-exist.
~たことがない is the negative of ~たことがある — 'has never had the experience of X-ing.' The に on 以前 marks it as a temporal locator ('at any prior time'). The polite ~ません closes the negative existential ある — fitting for a careful, formal reply or a third-party report. Compare casual それを見たことがない, which has the same structure but drops the polite layer.