Howard Williams on Thursday said when Ms Howell and her then-boyfriend Paul Charlton visited him in Horsham, months before she was found dead at the bottom of her home's stairs in April 2007, the couple had a heated fight in their car.
The argument, Mr Williams said, resulted in the 51-year-old woman throwing a hamburger at Charlton.
"Paul then said to me: 'I can't take this any longer, I'm 'gonna f***ing kill her," Mr Williams told Melbourne Magistrates Court.
Charlton, 67, was charged with Ms Howell's murder in January last year and is facing a week-long committal hearing to determine whether he stands trial before the Victorian Supreme Court.
One of her best friends, Toni Perryman, said Ms Howell told her Charlton had hit her within three months of them seeing each other.
"It was such a dysfunctional relationship," Ms Perryman told the court on Thursday.
"I would swear on my kids' heart that no one will else was responsible for Joanne's death than Paul Charlton."
The court has heard from multiple friends this week who all believed Charlton was responsible, including Catherine Tormey who on Wednesday said she knew something wasn't right when she saw Ms Howell the day she died.
Ms Tormey was so concerned after the visit she invited Ms Howell to come over for dinner and stay the night, but the 51-year-old eventually called back to say everything was fine.
Jade Bamber, whose mother Claire Bamber was also friends with Ms Howell, said her home looked like it had been "ransacked" when the pair went to help clean it up after her death.
Charlton was arrested at his home in Casterton, in western Victoria, a week after police launched a fresh appeal for information, describing Ms Howell's death as a solvable crime.
The hearing continues.
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