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She means nothing to him.
Literal
She [topic-は] him [for-にとって] nothing-at-all person is.
なんでもない ('nothing, of no significance') is a set phrase — 何でもない literally 'is anything not' = 'is nothing.' Used to dismiss the importance of a person, thing, situation. As a romantic statement, the contrast with declarations like '最も大切な人' ('the most important person') or '全て' ('everything') makes this phrasing land as a deflating reversal — what looks important to one might be nothing to another.