When she wrote, she did not know the extent of what Weatherston had done, the young woman told defence counsel Judith Ablett-Kerr, QC.
"I was in shock," she said.
She thought there might have been a mistake of some kind, and she "couldn't believe it" when she found out the details, she told the court.
Weatherston (33), a former University of Otago economics tutor, stabbed or cut 22-year-old Sophie Elliott 216 times during an attack in her bedroom soon after noon on January 9, 2008.
He has pleaded guilty to manslaughter but denies murdering Miss Elliott.
The trial was in its seventh day when it had to be adjourned about noon yesterday because a juror became ill and had to be taken to hospital.
The witness,whose identity is suppressed, was in a relationship with Weatherston for about three years until mid-2007, about the time he became involved with Miss Elliott, who was then one of his students.
Cross-examined by Mrs Ablett-Kerr, the woman read her undated letter to Weatherston.
In it, she urged him to "dig deep" and use his coping skills, assuring him he had many people who cared about him. She said she would write to him again and sent him notepaper so he could write to her, but warned him to be aware everything he wrote was likely to be read.
She said she would not be able to visit him that week as she had to work.
"In the event, you didn't visit him in prison, did you?" Mrs Ablett-Kerr said, asking if that was because the young woman became a witness for the Crown.
The woman said her status as a Crown witness was "part of it" but "a combination of things" were behind her not going.
"When I wrote the letter, I didn't know the extent of what he had done," she said.
Learning what the injuries were "finished it".
She "couldn't believe it", the witness said.
She also read to the court the letter Weatherston sent her in reply to her undated letter.
His letter, dated January 14, 2008, was on the notepaper she sent him.
Earlier, the witness told the court living with Weatherston could be "a bit like walking on eggshells".
During her time with Weatherston, she learned she had to be "quite careful" with him, that if she said something that set him off he would "really go off".
But their relationship was generally loving and kind, although she found it stressful when he came under stress.
"He had two sides - a loving and generous side and a nasty, mean side which he seldom showed in public."
While in the relationship, she never challenged Weatherston nor questioned his sexual performance.
She would not have compared his sexual organs to anyone else's, although she did once "reluctantly" when he asked her directly.
She never implied he was "a retard", but Weatherston told her Miss Elliott had called him that.
"I thought she was probably saying it in jest and I suggested that to him.
I said I didn't think it was directed to his intelligence or meant that way.
But he took it differently, and referred to it several times," the witness said.
She knew he had been seeing a psychotherapist for years and he was on Prozac for depression.
"You knew he was psychologically fragile?" Mrs Ablett-Kerr asked.
The witness agreed there was "an element of fragility" to his personality.
She recalled two academic upsets when Weatherston believed two people had taken his ideas and not given him credit.
He was "quite distressed" over it, and the witness said she was aware he took one case to extreme lengths, involving a mediator.
After their relationship ended and she had left Dunedin, they remained close friends and had regular phone contact.
Towards the end of 2007, the pattern of his calls involved a lot of moaning about Miss Elliott.
"He said he couldn't believe how horrible she was and how he had let someone so negative into his life."
He would go "on and on" about his application for a lecturing job and his concerns that what Miss Elliott was saying to a senior staff member might have affected his job prospects.
The witness agreed, at some stage, she had referred to Miss Elliott "in jest" as ROGS the rotten other girlfriend.
But she would only say that when Weatherston was upset about Miss Elliott, and she did not mean it in a nasty way.
She agreed Weatherston was concerned with his looks, but she was unaware that he had consulted a cosmetic surgeon.
At his graduation in December 2007, Miss Elliott was taking photographs and was acting "in a fairly volatile way, speaking loudly over everyone else".
"I thought she was quite nervous," the witness said.
She agreed Miss Elliott appeared image-conscious.
When she last saw Weatherston at New Year last year, she knew his relationship with Miss Elliott was over and he had a new girlfriend.
He seemed happy, so it was a surprise when he telephoned her on January 7, clearly upset and recounting an incident in which he said Miss Elliott had assaulted him that day.
"He went on and on about how upset he was . . . He said he couldn't believe she'd had such a negative influence on his life, that she had sucked his strength and he was just waiting for her to leave."
