She's his wife? Well, well — you're quite the dark horse, aren't you.
Literal
She [topic-は] wife? You [also-も] corner [in-に] not-placeable [feminine-わねえ].
隅に置けない ('can't be placed in a corner' = 'not someone to underestimate, full of hidden depths') is an idiom expressing surprised admiration at someone's hidden accomplishments — here, romantic ones. わねえ is the drawn-out feminine ね expressing teasing admiration. The sentence captures a gossipy moment of delighted surprise.