What is Jack's character like, and where does he fit in to the story?

'Jack' in the story is Sergeant Jack Noonan, a police
officer known to Mary Maloney as he worked alongside her husband Patrick. Jack is a foil
to Patrick Maloney and Mary Maloney's characters. He is caring and compassionate towards
Mary-

Dahl also uses some of Jack's dialogue to illustrate
the irony of the investigation into the attack on Patrick
Maloney-

“It’s
the old story,” he said.  “Get the weapon, and you’ve got the
man.”

There is
great irony in the fact that the officers keep overlooking the murder weapon as it is in
the oven. Also, they are not looking for a man, as it is Mary who is
guilty.

Mary uses Jack's sympathy for her to her advantage.
She asks him to get her a drink, and convinces him to talk the other men in to eating
the leg of lamb.

 Jack's naivety concludes the scene of the
investigation at the end of the story. The officers discuss the whereabouts of the
murder weapon as they eat it-

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“Probably right under our very noses.  What you
think, Jack?”

And in the other room, Mary Maloney began to
giggle.