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Paul Wilson (55), also known as Paul Tainui, has pleaded guilty to raping and murdering Christchurch 27-year-old Nicole Tuxford in Merivale last year.
He raped and murdered his former girlfriend, Kimberly Schroder (21), in 1994.
Wilson met Mr Bain in jail and was groomsman at his 2014 wedding.
Joe Karam, long-time advocate for Mr Bain, said yesterday Mr Bain was distressed Wilson inflicted such a disaster on the victim and her family.
He said Mr Bain thought his friend was managing life well.
Wilson lay in wait for eight hours overnight, before Ms Tuxford arrived back in her Merivale home in April last year.
At the High Court in Christchurch yesterday morning, Wilson has pleaded guilty to sexual violation.
He had earlier pleaded guilty to the life coach's murder.
Justice Cameron Mander has indicated a sentence date of March 28.
The news in April last year Wilson had committed a second murder - 24 years after he murdered Ms Schroder in Hokitika - was too much for Ms Schroder's father.
Gary James Schroder (67) died in a suspected suicide just hours after Wilson made his first court appearance on April 10 last year.
When Mr Bain wed primary school teacher Liz Davies at a winery outside Christchurch in January 2014, Wilson was his groomsman.
Mr Bain and Wilson met while working in the Christchurch Men's Prison kitchen, a former inmate who shared a cell with Mr Bain said.
The pair were two of the longest-serving inmates in their section of the prison and, though other inmates came and went, the two remained close.
''They sort of naturally became ... very close friends,'' Mr Karam said in 2014.
''The single person he [Mr Bain] has spent the most time with over the past 18 years is Paul Wilson.''