Readers know that Mary Maloney is very happy in her husband's presence because the story fairly overtly tells readers this information. When Mary is introduced to readers, she is patiently awaiting her husband's return from work. She greets him at the door, takes his coat from him, and returns with a drink in hand, serving his every need. In the following paragraph, readers are then told how much Mary loves the following moments when she gets to just sit down and "luxuriate" in Patrick's presence.

For her, this was always a blissful time of day. She knew he didn’t want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone in the house. 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.

We don't know why Patrick gives her the news that he does, but it is possible that he feels smothered and overly confined in Mary's presence; however, it is still shocking to readers that he could so coldly end a partnership with someone that so clearly adores him. It's also quite amazing that Mary is able to move so quickly from practically worshiping the ground Patrick walks on to killing him, establishing an alibi, and disposing of the murder weapon.