。
She went there alone.
Literal
She [topic-は] there [to-へ] alone went.
ひとりで ('alone, by oneself') uses the noun ひとり ('one person') with the manner-marker で — literally 'with one person', i.e., 'with no one else'. The で here is the で of accompaniment-or-state, parallel to 笑顔で ('with a smile') or みんなで ('all together'). Note that ひとりで differs from 自分で ('by oneself, of one's own accord'): ひとりで emphasizes solitude (no one with her), 自分で emphasizes self-agency (without being made to). 'Going alone' here uses the former.