Claire Brown told the jury at Chester Crown Court that her friend, Jenny Cupit, was upset when she learnt Kathryn Linaker, a deputy head teacher, was pregnant.
Mrs Cupit, a former hairdresser of Borrowdale Avenue, Warrington, has pleaded not guilty to murdering Mrs Linaker, 33, in front of her four- month-old baby at her home in Penketh on 17 April last year. The mother- of-two, who is 24, has admitted manslaughter on the grounds of diminished responsibility but the Crown is contesting her plea.
Mrs Brown met Mrs Cupit and her husband Nick at the Centenary Operatic Dramatic Society, where Mrs Linaker and her husband Chris were also members. Mrs Cupit and Mr Linaker began a relationship in January 1997. Mrs Brown told the court that Mrs Cupit had confided in her about the affair.
She said: "Jenny started crying, and she said Kathy was pregnant. Jenny was crying a lot but she made it clear that she was seeing Chris. I said she should leave them to it."
Mrs Brown told Alex Carlile QC, for the prosecution, of a telephone call just weeks before her death: "Jenny was upset, saying that Chris wasn't going to leave; that she hated Kathy because she had something that Jenny didn't. She said Kathy had a perfect family and that hers was breaking up."
Under cross-examination, the witness told Adrian Fulford QC, for the defence, that she was worried about the effect the affair was having on her friend.
The defendant's mother-in-law, Barbara Cupit, told the jury she believed the defendant had a split personality. She said: "Jenny was a very affectionate girl but could be very hurtful."
The court then heard the tape of Mrs Cupit's interview with Detective Sergeant Ian Hughes at Warrington police station after the attack.
On the tape, Mrs Cupit said: "She [Mrs Linaker] said she hated me even if I wasn't involved with Chris. He paid me more attention than her.
"She came out of the kitchen with a knife. I grabbed the knife. I didn't mean it - I love her."
Mrs Cupit described to the officer how Mrs Linaker became angry: "I told her to calm down, and that we would talk about it and sort it out. She was really starting to scare me. She came at me with a small knife.
"I turned and grabbed her, and cut my hand on the knife."
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