If she is not an exceptional woman, she will be made happier by his affection for the mother to whom she is devoted, and miserable by a lack of this sentiment.

Let us argue the case according to rule.

It makes Mary happy if John is fond of her mother, and unhappy if he is not.

If John loves Mary he wishes to make her happy.

_Ergo_, when he shows his love for her mother he is likewise giving evidence of his love for Mary.
So, when I hear a so-called devoted wife cast unkind slurs upon her mother-in-law, I wonder how genuine is the affection for her husband which allows her to make him unhappy by awaking in his breast suspicions that his mother is distasteful to his wife.