。
She wrote about it in her diary.
Literal
She [topic-は] that thing [object-を] diary [in-に] wrote.
~を~に書く reverses what English instinct expects: the *content* takes を (what's written), the *medium* takes に (where it's written into). 日記に ('into the diary') marks the destination of the writing — に for the surface or container the words land on. そのこと ('that thing, that matter') uses こと to wrap a previously known matter into a single noun, often pointing back to context. Diary-keeping has a long tradition in Japanese literature, from the Heian-era 紫式部日記 ('Murasaki Shikibu Diary') and 蜻蛉日記 ('Kagerō Diary') to modern 日記文学 ('diary literature') — the introspective written record is a deeply embedded literary form.