She looked for friends but couldn't find a single one.
Literal
She [topic-は] friends [object-を] searched [but-が] anyone one-person did-not-find.
誰一人見つからなかった — 誰一人~ない is an emphatic 'not a single person.' 見つからなかった is the negative past of the intransitive 見つかる ('to be found'). The が connects the search attempt to its failed result.