From all textual evidence, Mary Maloney has been a devoted wife until the night of this exchange. She anxiously awaits her husband's return home from work. She takes his coat and puts it away for him. She makes his favorite drink and knows that he wishes to drink it in silence, so she patiently awaits his coming home so that she can spend time with him. And she adores him—the way he sits in his chair, the look in his eyes, and even his need for quiet:
She loved to luxuriate in the presence of this man, and to feel—almost as a sunbather feels the sun—that warm male glow that came out of him to her when they were alone together.
Even more, Mary is six months pregnant with their child.
And on this particular night, Patrick has decided that he is going to leave his devoted and very pregnant wife. Dahl writes:
And he told her. It didn't take long, four or five minutes at most, and she sat very still through it all, watching him with a kind of dazed horror as he went further and further away from her with each word.
The reason that the dialogue Patrick uses isn't included is because it is completely insignificant. Patrick prefaces his statements by telling his wife that this news will be shocking to her, meaning that she could not have possibly seen this coming. She holds no fault in the ending of the marriage. He has simply made other choices for his future—and she is not part of that future.
So whether Patrick is simply tired of his wife or is having an affair or wants to live as a recluse simply isn't significant. What is significant is that his devoted wife has done everything she could have done in order to be loved by her husband, and he decides to leave her without any hope of reconciliation and without any cause.
Mary could easily become a victim here, but in the end she finds the strength to end her marriage on her own terms, and the story ends with her giggling about her successful revenge.