。
He had one daughter.
Literal
He [to-には] daughter [subject-が] 1-person existed [polite-ました].
彼には is the dative possessor construction: the に marks the possessor, は adds topical contrast. Japanese often says 'to X, there exists Y' instead of 'X has Y.' 一人 is the counter for one person with the special reading ひとり.