"I called her a murderer," Adam Kearns told the High Court at Christchurch.
"I knew what she was doing. I basically said, 'You're sick, you're sick in the head. I don't want you as my mother any more'."
Milner denies murdering Phil Nisbet, 47, on May 4, 2009 in a case that police originally ruled suicide.
Mr Kearns - then aged 18 - told the court his mum had talked several times about killing Mr Nisbet, even discussing methods, including putting crushed glass in mashed potatoes and poisoning him with drugs and sleeping pills.
Initially, he "laughed it off - along with everyone else".
"I didn't think my own mother would be capable of that. I didn't think she'd have it in her to do it."
The talk of getting rid of Mr Nisbet started out "very subtly", but became more concerted and more detailed over time, Mr Kearns said.
The night he stumbled across his mum allegedly crushing up a green/blue powder on the kitchen bench and putting it into clear capsules, he said she looked "shocked to see me... a guilty look."
She then broke down in tears, and allegedly told him, "I'm not going to do it, I'm not going to do it."
That night, April 15, 2009 Mr Nisbet was admitted to hospital for a second time that day, feeling unwell.
The delivery driver thought he'd suffered an allergic reaction to a spider bite.
He was "pale as a ghost" that night, Mr Kearns said.
The Crown claims Milner had just tried to murder him.
Milner denies murdering her second husband by giving him a fatal overdose of the antihistamine and sedative Phenergan, and possibly finishing him off with a pillow over his face.
The Crown said she was unhappy in her marriage and motivated to murder by the prospect of cashing in the $250,000 life insurance policy.
Milner plotted the best ways to kill her husband; buying drugs under false names, asking friends for views on poisoning methods, and even offering to pay $20,000 for a hit man to kill Mr Nisbet, it is alleged.
Within days of allegedly seeing his mum with the crushed up powder, Mr Kearns moved out of the Christchurch home and told Mr Nisbet that Milner was trying to kill him.
He laughed it off, he said.
Within days of Mr Nisbet's death, Mr Kearns went to police with his concerns.
"I couldn't live with it any more," he said.
He gave them a cellphone and drew their attention to a series of text messages between him and his mum.
The jury heard a series of messages where Mr Kearns said Milner was talking about "getting rid" of her husband.
"I told him 'you're going to move out', he got all shitty. Might go into after hours soon, I've had enough," she apparently said to Mr Kearns in a text on April 14, 2009 - the day before the Crown says she first attempted to kill her husband.
Crown prosecutor Brent Stanaway asked Mr Kearns what the 'after hours' reference meant.
"I think that was talking about the chemist," he said.
The next day, she allegedly warned her son, "You can't tell anyone what I want to do."
Asked what that meant, Mr Kearns says it was a "threat", and wanting him to help cover her tracks.
The trial, before Justice David Gendall, continues.
- Kurt Bayer of APNZ
