Jessica Smith, who had known Sophie since both were in year 7, was giving evidence in the trial of 33-year-old Weatherston, a former economics tutor and part-time lecturer charged with murdering Miss Elliott (22) on January 9 last year.
Weatherston admits killing her but denies the charge of murder.
She was stabbed 216 times during an attack at her home.
The defence says Weatherston lost control because of provocation from the emotional pain caused by the relationship.
His trial before Justice Judith Potter and a jury goes into its fifth day of hearing today in the High Court at Christchurch.
Miss Smith said she made Miss Elliott promise her she would not go to see Weatherston to pay him for damage she earlier caused to a door at his apartment.
"I was so scared after what he had done the last time. But she wanted things resolved," Miss Smith said.
Sophie had told her of visiting the accused on December 27 to give him a book of photographs of his graduation celebrations.
When she made it clear the relationship between them was over, he had changed from saying he loved her to abusing her.
He grabbed her, threw her on his bed, straddling her and pushing his arm against her throat.
She managed to escape and run from the flat but Weatherston followed, yelling at her that he wished her plane had crashed when she was returning from a holiday in Australia in November.
In a text message on January 8, Sophie told her about meeting Weatherston by chance the previous day.
He had called her "mentally deranged", Miss Smith said.
Sophie had also told her Weatherston made hurtful remarks about her appearance and intelligence and accused her of being spoiled and "living off her parents", something Sophie found hurtful.
There were times when she did not know if the relationship between the pair was "on or off".
"I just knew it was up and down," Miss Smith told the court.
Asked if Sophie had discussed the sexual aspects of the relationship, she said Sophie indicated there were difficulties.
