Nurse Copling is a nurse who took care of Mary Pritchard.
Nurse Copling changed Mrs Pritchard's smelling salt with cyanide, thus killing her.
In the ITV 2010 adaptation of the novel (Episode 3, Season 5 of Agatha Christie's Marple), Nurse Copling is played by Claire Rushbrook. Here she is given the firstname "Caroline". The method of the killing is the same, but the motive is changed somewhat. Here it is more straightforwardly about money. She had calculated that with Mary dead and George framed for the murder, the wealth would devolve onto George's brother Lewis Pritchard and his wife Philippa Pritchard. She had learnt about Philippa poisoning Mary (though just enough to make her sick without being lethal) out of spite that Mary had taken George from her. Copling blackmailed Philippa into agreeing to hand over half of the inheritance in exchange for not reporting her to the police.
In the NHK adaptation The Blue Geranium, Episode 15 of their anime series Agatha Christie's Great Detectives Poirot and Marple, Nurse Copling is portrayed in much the same way as in the original story. Here, after confessing to the crime, she tells George that she did it for him, because she could not bear to see him suffer because of his demanding and unreasonable wife.