Pamela Pearson, 40, said Philip Wade came to see her at her home and told her he was having nightmares and not sleeping well.
She was giving evidence at the trial of Wade, 42, Colin Coats, 42, David Parker, 38, and Paul Smith, 47, who deny abducting, torturing and murdering missing financial adviser Lynda Spence, 27, who disappeared in April 2011.
The four also deny attempting to defeat the ends of justice by disposing of, destroying or concealing Ms Spence's body.
"He told me he hadn't been sleeping very well because him and Colin had disposed of a woman's body," Ms Pearson told jurors.
Solicitor General Lesley Thomson, prosecuting, referred the witness to a statement she gave to police on October 27, 2011.
The statement read: "I asked him why (he had been having nightmares) and he said, 'I've helped dispose of that woman's body'. I asked him what he meant by that. (He said) 'I was forced to dispose of her.'
The statement went on: "I asked if it was the woman whose phone was found in Kilbirnie and he said it was. I asked how her phone ended up in Kilbirnie, and he said it was a mistake."
The court had heard that a mobile phone used by Ms Spence was found in a bin outside the Lunchbox cafe in Kilbirnie, Ayrshire, on June 2, 2011.
The statement said: "(Mr Wade) said 'me and Colin disposed of her in the Glasgow hills'."
Ms Pearson said in court: "I do remember that."
Earlier, the court heard that Wade allegedly told another friend that he Tasered someone who, it was claimed, had come to "do him and Colin in", and chopped and bagged him up.
Mark Asbury, 47, said Wade told him they had intercepted the man, referred to as "some scouser" in the stairwell of Wade's flat in Glengarnock, Ayrshire.
Under cross-examination by Gary Allan QC, defending Wade, Mr Asbury agreed that he had previously lied to police about not having seen a silver Vauxhall Astra which, the jury has heard, Ms Spence rented.
Mr Allan said he was a person who would "tell lies at the drop of a hat" to protect his own interests.
The trial before Lord Pentland at the High Court in Glasgow continues.