。
She'll make him a fine wife.
Literal
She [topic-は] his [genitive-の] good wife [into-に] become [probably-だろう].
~だろう marks the speaker's conjecture or probability — 'probably, likely.' It's the casual counterpart of でしょう (polite). The ~になる pattern ('become Y') is the intransitive partner of ~にする ('make Y'); used here for the natural progression into a future role. Note: 妻 ('wife') in modern Japanese has multiple variants (妻, 家内, 奥さん, etc.), each with a different politeness/contextual use; 妻 is the neutral, current default.