Jacqueline Watt, 38, said David Parker originally told her he was "babysitting" a man but he later admitted it was Ms Spence, who disappeared in April 2011.
Ms Watt yesterday told the High Court in Glasgow that financial adviser Ms Spence had arranged a mortgage for her in 2009. She described Ms Spence as a "dodgy broker" and said she and Parker were watching publicity about Ms Spence's disappearance on the news.
She told the court: "I said, 'that's the lady that got me my mortgage, do you think she's either bumped the wrong person or done a runner because she owes so much money?'.
"David was quite agitated and he said, 'that's the him'. He had told me [previously] he was babysitting a man who was in trouble with money, keeping some man safe in a house. But he said it wasn't his house.
"I asked him, 'what do you mean that's the him?'. He said, 'it wasn't a him, it was a her'.
"He was saying, 'you don't understand, I can't tell you'. He was upset, agitated, scared. He was in fear.
"I just thought he was talking gibberish."
Ms Watt also said Parker accused her of wearing a wire, but she had told him, "you've been watching too much CSI".
She claimed the conversation about "babysitting" took place in April or May 2011, and Parker told her about it being Ms Spence about a week before he was arrested in connection with her disappearance.
The court also heard Ms Watt visited Parker in Barlinnie prison on December 23, 2011, when he gave her information he wanted passed to the police or his lawyer. She said she noted some of what was said on the back of an envelope.
Parker, 38, Colin Coats and Philip Wade, both 42, and Paul Smith, 47, deny murdering Ms Spence, 27.
The trial before Lord Pentland continues.