The convicted killer known as the "Black Widow" had a "no strings attached" relationship with a man who went on to bash and strangle a woman to death.
Police say Helen Milner asked them to phone a mystery friend called Keith the morning her husband was found dead in bed at their Christchurch home in 2009.
The Press can reveal he was Keith Donald Bonner, who three years later killed Tracey Lee Morris and left her body in a car.
In a formal witness statement to police, Bonner said he met Milner through a dating agency more than 15 years ago. He described their sexual relationship as "no strings attached" and "friends with benefits" before it ended in either 2000 or 2001.
The revelation is the latest in a string of bizarre details to emerge since Milner, 50, was found guilty last month of the murder and attempted murder of her husband, Philip James Nisbet.
Nisbet, 47, a truck driver, was found dead in bed at the couple's home in Checketts Ave, Halswell, on May 4, 2009.
Police initially believed his death was suicide and referred the matter to the coroner.
However, after an inquest, they launched a homicide investigation and Milner was arrested in October 2011.
The Crown said Milner likely mixed the drug Phenergan with her husband's dinner and then may have suffocated him once he was sedated.
She was motivated by his $250,000 life insurance policy.
During the three-week murder trial at the High Court in Christchurch, the jury heard Milner asked police to contact a friend called Keith on the morning Nisbet's body was found.
Lee-Anne Cartier said she was shocked to hear her brother's killer had previously been in a relationship with Bonner.
"It just revolts me that's the first person she [Milner] turned to," Cartier told The Press.
"It just really reinforces her character and who she is - an evil, horrible being.
"Their unhappy ever after will be spending the rest of their lives rotting in neighbouring cells."
In July 2012, Bonner, 55, was jailed for a minimum of 13 years for murdering Morris, a woman he described as "his greatest friend". They had known each other for 23 years.
The 46-year-old mother and bar worker at Papanui was found dead in her car near Christchurch Airport in January that year.
She was naked from the breasts down, with her leggings wrapped around her neck.
The Press has previously revealed Milner was cut from her mother's will and disowned after years of lies and deceit that tore her family apart.
During last year's high-profile murder trial, she was already a serving prisoner after she framed her son, Adam Kearns, for a crime he did not commit.
She is scheduled to be sentenced on February 20.
- Fairfax Media