。
It was the first time she ever betrayed a friend.
Literal
She [topic-は] for-the-first-time friend [object-を] betrayed.
裏切る ('betray') is built from 裏 ('back, reverse side') + 切る ('cut') — literally 'to cut from behind,' a vivid image of stabbing someone in the back that mirrors the English idiom almost exactly. はじめて as an adverb means 'for the first time'; the related ~て初めて pattern (~て初めて気づいた, 'only after X did I realise') uses the same word in a different syntactic frame. The placement of はじめて between topic and object makes it the highlighted information — the noteworthy point isn't that she betrayed a friend but that this is the first such instance.