The point of view is strictly that of Mary Maloney. Here is a typical example.
For her, this was always a wonderful time of day. She knew he didn't want to speak much until the first drink was finished, and she was satisfied to sit quietly, enjoying his company after the long hours alone in the house. She loved the warmth that came out of him when they were alone together. She loved the shape of his mouth, and she especially liked the way he didn't complain about being tired.
The narrator tells what Mary thinks, feels, and does. When Patrick is described, it is through Mary's point of view (POV). For example:
When he came back, she noticed that the new drink was a very strong one. She watched him as he began to drink.
The reader is not permitted to enter into Patrick's mind but can only guess what he is thinking and feeling from the way he acts, as observed by his doting wife. We know that Patrick has something important and painful on his mind because Mary sees he has made a very strong highball.
There is an excellent anthology of short stories arranged by points of view as classified by the editors, James Moffett and Kenneth R. McElheny. The full title of the book is Points of View: An Anthology of Short Stories (Rev. Ed. August 1995). "Lamb to the Slaughter" is not included in this anthology, but the editors would place it in the category they call ANONYMOUS NARRATION--SINGLE CHARACTER POINT OF VIEW. This is a very common story-telling technique. The story is told in the third person, e.g., "She watched him as he began to drink." The way Roald Dahl handles point of view in this story shows he is an accomplished fiction writer.
Point of view is very important in story-telling because it is the chief way in which the author gets the reader emotionally involved with one of the characters. In "Lamb to the Slaughter" we sympathize and identify with Mary Maloney, even though she commits a murder, because we are held in her point of view from beginning to end. There is no one else with whom to identify without switching points of view, which can risk losing reader involvement. The other important way in which the author usually gets the reader to identify with one character is by giving that character a problem that must be solved. Mary's problem is that she has killed her husband and has to establish an alibi and dispose of the murder weapon.
When the investigating police officers are devouring the thoroughly cooked and delicious murder weapon, they converse among themselves; but the author makes it clear that their dialogue is all being heard by Mary and that the reader is still in her point of view. Here is part of the dialogue and the very last line of the story which shows that Mary is listening to the whole conversation:
"Personally, I think the weapon is somewhere near the house."
"It's probably right under our noses. What do you think, Jack?"
And in the other room, Mary Maloney began to laugh.