The answer to this question is subjective, and it is best left up to individual readers to interpret. I think leaving the conversation up to the imagination of each reader makes it very powerful. Reader imagination is a powerful thing, because more often than not, the reader takes things to a level beyond what a writer might write. Take horror/thriller movies like Alien or Jaws as an example. Audiences are shown very little of the monsters, so our imaginations work toward building the creature into a horrifying menace. By not letting us know exactly what Patrick says, the narrator is leaving it up to individual readers to guess. That forces each reader to personalize the conversation. This forces male readers to figure out how and what Patrick said to make Mary react that way, and it makes female readers wonder what Patrick's words must have been to make her snap like that. The end result is the same regardless of the gender of the reader, but the imagined conversation might be quite different for each of those reader groups. An unwritten conversation gives the story that ability.
The author's main reason for handling Patrick Maloney's announcement in the way he does was probably that he wanted to avoid a dialogue between husband and wife. Mary would be more than likely to argue, to plead, to explain, to ask for explanations, and, of course, to bring up the subject of her expected baby. The impact on Mary would not be as decisive, and the end result would be the same. Patrick wants out of a boring marriage. This fact is essential to the rest of the story. His cold, brutal announcement leads, understandably, to his being clobbered with a frozen leg of lamb.
Mary's behavior before Patrick makes his surprising announcement was intended to illustrate dramatically what it is about her that makes her husband want out of their marriage. She doesn't realize that she is giving him claustrophobia. Any counter-arguments that Mary could make would only soften the impact of Patrick's cold, hard, irrevocable decision to walk out on her. His decision has to be irrevocable in order for her reaction to be, not only credible to the reader, but also sympathetic.