。
She is irreplaceable to him.
Literal
She [topic-は] he [from the perspective of-にとって] irreplaceable person is.
にとって sets the perspective — 'from his point of view' — distinct from a bare に, which would mark him as a recipient or target. かけがえのない is a fixed phrase literally meaning 'having no substitute,' attached to 人 ('person') as a noun-modifier via the genitive の. The plain copula だ keeps the register declarative and casual.