。
She'll probably make a good match for him.
Literal
She [topic-は] his [genitive-の] good partner [into-に] become [probably-だろう].
相手 ('partner, opponent, the other party') is one of those usefully ambiguous Japanese words — covers anyone you're paired with, whether romantically, in conversation, in negotiation, or in conflict. いい相手 ('good partner') here is romantic/marriage-coded. The ~になるだろう pattern projects the future — 'will probably become.'