The senior constable has pleaded not guilty to the murders of Jesse Baird and Luke Davies, suggesting the two men died after two separate struggles in Mr Baird's house.
Lamarre-Condon entered the Paddington house in February 2024 armed with his police pistol and heavily disguised.
But he did not intend to kill his former partner, Mr Baird, and did not shoot the other man, his barrister has told a NSW Supreme Court jury.
The jury on Friday was told that after Mr Baird ended their casual relationship for a second time in November 2023, Lamarre-Condon used a friend's Instagram account to send a 1600-word message to the TV presenter.
The message claimed a tumour in Lamarre-Condon's shoulder could mean his arm needed to be amputated.
It was sent from Shae Davison's account so as to appear it came from the close friend, rather than Lamarre-Condon, the jury was told.
But it was written by the senior constable, Ms Davison said on Friday.
Lamarre-Condon did not have cancer and was not being treated as such.
The message said Lamarre-Condon had been prescribed a number of strong medications 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.
"So far only (another friend) and I know about this … he hasn't even told his mum, and he doesn't want to freak her out," the message, purporting to be from Ms Davison, read.
She said she had known Lamarre-Condon for about 17 years at the time.
Ms Davison said she pushed back on the proposal to send a message off her account.
Lamarre-Condon insisted she not read the message before it was sent, she told the jury.
"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."
Ms Davison told the jury about her interactions with Lamarre-Condon towards the end of his fluctuating relationship with Mr Baird.
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
The implication was false and Mr Baird could not see the post - only those Lamarre-Condon believed to be romantic rivals could.
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.
"He said he staged it," she said.
Lamarre-Condon also admitted going on Mr Baird's phone, deleting Instagram messages and blocking some accounts, Ms Davision said.
She said she encouraged him to tell Mr Baird as their relationship struggled.
"I said if you continue to lie it's going to break down," Ms Davision told the court.
"If you come clean, there's a chance you can rebuild."
The trial continues.
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