Georgia Cook was one of two long-time friends of the police officer to testify in his NSW Supreme Court trial on Friday.
The senior constable has pleaded not guilty to the murders of Jesse Baird and Luke Davies, suggesting the men died after two separate struggles in Mr Baird's Sydney terrace in February 2024.
Ms Cook recalled a conversation she and Lamarre-Condon had over Thai food in a Kirribilli restaurant in May 2023.
"What would you do if I told you I killed somebody?" Ms Cook told the jury he said to her.
"Would you help me hide the body?"
Ms Cook, who had known Lamarre-Condon for more than two years at the time, said he laughed off the comment.
"No, I don't want their ghost to haunt me," she says she replied.
Lamarre-Condon then told her not to worry.
"Don't worry, I didn't do that - but I did do something crazy."
The "crazy" thing referred to involved breaking into Mr Baird's house, Ms Cook said.
Jurors have been told the police officer and Mr Baird had been in a relationship in the months leading up to April 2023, though the men had different views of its seriousness.
Ms Cook said the officer told her how he'd been surveilling his ex and saw Mr Baird take photos of other men with a Polaroid camera gifted by Lamarre-Condon.
While parked outside the Paddington home and seeing Mr Baird use a spare key, Lamarre-Condon used the key to enter when he knew it to be vacant, Ms Cook said the officer confided in her.
Once inside, he stole the camera, ate an Easter egg and "did something else he wouldn't tell me", she told jurors.
As well as contesting two murder charges, Lamarre-Condon denies breaking into Mr Baird's home in August 2023 while disguised.
Mr Baird ended a brief reignition of their casual relationship in November 2023, resulting in the TV presenter receiving a 1600-word message on Instagram from a friend of Lamarre-Condon, jurors heard on Friday.
The message claimed a tumour in Lamarre-Condon's shoulder could mean his arm needed to be amputated.
But while it came from Shae Davison's account, she testified on Friday it was written by the senior constable after he pressed his long-time friend to allow him to use her account.
"He put a lot of pressure on me ... he was so upset, he wore me down," she said.
"It felt like guilt-tripping to me, and using our friendship to get what he wanted out of that."
The message said Lamarre-Condon had been prescribed strong medication in preparation for chemotherapy or radiation before a surgery to cut out the so-called tumour in early December.
Stating the medications were the "strongest possible" ones, the message said they had "thrown off" Lamarre-Condon's hormones and helped explain how he had behaved day-to-day.
Around the same time, Lamarre-Condon admitted posting an Instagram story with an internet picture of a bunch of flowers and a message implying Mr Baird had sent them, Ms Davison told the jury
Ms Davison said when Mr Baird found out about the ruse, Lamarre-Condon got an identical bouquet and took a photo of the flowers with his mother, telling Mr Baird the post referred to her.
Lamarre-Condon's barrister has conceded his client did some "stupid things" but disputes he exhibited threatening behaviours before the fatal incident in the Paddington terrace.
"He never planned to kill or ever cause any harm to anyone," John Stratton SC has told jurors.
"It was in self-defence."
The trial resumes on Monday.
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