Now 22, Mr Kearns said he had no sympathy for the woman found guilty in a unanimous verdict yesterday of murdering Mr Nisbet.
"She always told me when I was a kid that if you do the crime, you do the time. She always said that," he said. "Now it's me turning round and saying the same thing back to her."
A jury of seven men and five women delivered their verdict on 50-year-old Milner after seven-and-a-half hours of deliberations over two days in the High Court at Christchurch.
They found her guilty of drugging Mr Nisbet, 47, with the sedative Phenergan on May 3, 2009, then, when he was unconscious, probably finishing him off by smothering him.
They also found her guilty of one charge of attempted murder, but not guilty of a second.
There were sighs of relief in the public gallery at the guilty verdicts and Milner was teary in the dock as she was led to the cells.
Outside court, Mr Nisbet's youngest brother, Andrew, addressed the media on behalf of the family.
"Finally, some justice for my brother after over four-and-a-half years. We now want to move on and look forward to a sentence that's going to be appropriate to this terrible crime."
The Crown argued Milner faked her second husband's death to look like a suicide, motivated by a $250,000 life insurance payout. The defence argued it really was suicide.
Milner almost got away with it. A homicide investigation was launched two years later only after a coroner refused to find the death was suicide.
Mr Kearns said he knew his mother was a murderer because she had talked to him about killing Mr Nisbet for years.
The trial heard Milner even offered to pay his brother Greg $20,000 for a hitman to do the job.
Adam Kearns' girlfriend at the time, Kasey Woodstock, told the 14-day trial Milner offered them a slice of life insurance cash if they spent $5000 on hiring a killer.
Mr Kearns said he tried to warn his stepdad of her wicked intentions. "One day as I was leaving their house, I warned Phil. I said, 'She's trying to kill you, mate', and he sort of laughed it off. He was too naive.
"A couple of days after Phil died, I went to the police with my concerns. But they didn't take me seriously at all. They'd already put it down as a suicide."
Mr Kearns' decision to go to the police had unexpected consequences - his mother framed him for sending fake death threats, putting him into prison on his 19th birthday.
Milner was eventually jailed for two years and eight months for perverting the course of justice, but not before her son lost the last of his youth, his job and even his shoes.
She decided to get even by contacting Miss Woodstock - who split with Mr Kearns on bad terms - and convinced her they should both take out protection orders against him, claiming he was stalking and harassing them.
Mr Kearns was then taken into custody for sending his mum death threats via text message.
"I was arrested and charged with breach of protection order and threatening to kill, and put straight into prison," Mr Kearns said.
"I was like, 'What?"'
At his first court appearance the following day, bail was refused and he was remanded in custody at Christchurch Men's Prison.
He spent his 19th birthday behind bars.
While inside, the industrious teenager set about clearing his name. He phoned his lawyer and passed on his email account details and password, along with instructions about where to find an email he'd kept from months before.
It was a request from his mother asking him to make fake death threats for her. "I didn't do it, but I kept the email thinking it might come in handy at some point."
It was enough to get police checking out her story.
Officers traced CCTV from a shop that showed Milner buying the cellphone she used to set up her son.
She was arrested and charged, but it still took three days for Mr Kearns to be released from prison.
"I spent 18 days behind bars in total. That was my first time in jail. I got jumped in the paddy-wagon for my shoes. It had a long-term effect on me.
"While I was in prison, my flatmate sold parts off my car to pay my rent. I didn't blame him; he didn't know what was going on.
"I lost my furniture [and] a really good job I had as a kitchenhand at a restaurant in Sumner.
"She pleaded not guilty and straight away gets bail," he said.
Mr Kearns is suing his mother for damages. APNZ understands he's seeking $60,000.
"All I want is to get back what I lost," he said. His lawyer, Kerry Cook, said Mr Kearns was suing over a "malicious prosecution ... that she knew to be false".
Papers were served on Milner during her trial. She has yet to respond.
Mr Kearns is also disappointed with the police's initial response to Mr Nisbet's death and inability to see through his mother's deceit.
"I want everyone to know that the police f***** up, and for them to be held accountable," Mr Kearns said.
"The police could have dealt with me a lot better. I'm not a victim, as the son of an offender, and even with the text messages, I wasn't deemed to be a victim, so I've not had any help or counselling. I'm still drowning in the sea.
"Five years on, I'm still bumping from job to job because I've got other s*** on my mind."
Mr Kearns said it had been tough watching his mother tried for murder and giving crucial evidence against her, even though she "made my life hell" as a child.
"My friends would come over and she'd be nice as pie, but then they'd leave and she'd just turn on me. She had two completely different sides."
He hoped his mother would admit she had problems and get help but wasn't fazed by being the son of a convicted murderer.
"I've always prepared myself for it. I'd probably have had more mixed feelings if she hadn't set me up and put me in jail.
"But the way she's treated me all my life ... and has been as a person ... What goes around, comes around."
Milner will be sentenced in February.
What is Phenergan?
• Phenergan is an antihistamine which blocks the effects of a chemical naturally produced by the body
• It is used to treat allergy symptoms such as itching, a runny nose, sneezing, itchy or watery eyes, hives, and itchy skin rashes
• It can also prevent motion sickness, nausea and vomiting or pain after surgery and aid sleep over short periods of time
• Overdosing on Phenergan can result in drowsiness, convulsion, unconsciousness, hallucinations and difficulty in breathing for adults
