Alleged wife-killer 'cheated on fiancée during Paris tryst'
Brenda Grant, 49, said she went to Paris with Malcolm Webster, 51, in February 2006.
But Simone Banarjee, 41, told the court last week that Webster had moved into her four-bedroom house in January 2006 and they were engaged.
Ms Grant told the court she had not heard the name Simone Banarjee at the time and Webster had not mentioned her in e-mails he sent.
She also said Webster told her his first wife, Claire Morris, had died in a car accident.
Ms Grant said: "I remember clearly. He told me he was destined to be alone for the rest of his life."
Webster is accused of murdering Ms Morris in Aberdeenshire in May 1994 and pocketing more than 200,000 in insurance payouts after her death. He denies drugging her, deliberately crashing a car she was a passenger in and then setting fire to it.
Ms Grant, from Kansas City, Missouri, met Webster when he visited the US in August 1995. They met through work and he had been "very charming". A romantic relationship started during his visit and ended around September 1996, but she said they kept in touch and later met for the visit to Paris.
"I considered him a lifelong friend," she told the court.
Ms Grant said Webster would get "very emotional" when he spoke about his first wife and had told her his feet were burned when the car caught fire.
The American, who said she also went on a holiday to France with Webster with her three children in August 2001, also told the court their relationship was "on the road to be serious" in 1995 to 1996.
Ms Grant said Webster had told her in an e-mail he had been diagnosed with leukaemia and had his spleen removed.
She said he sent her a fax telling her he was getting married to a Felicity Drumm in about January or February 1997.
Webster, from Guildford, Surrey is also accused of trying to kill Ms Drumm, his second wife, in New Zealand five years later to obtain 750,000 from separate insurance policies. He faces a further charge of attempting to marry Ms Banarjee bigamously and inducing her to make a will leaving everything to him.He denies the charges against him.
Catherine Brown, 45, told the court earlier yesterday that Webster took her to Ms Morris's grave when they spent the weekend together at a hotel in Ballater, Aberdeenshire, in December 2005.
Ms Brown, a nurse, told the court Webster said Ms Morris died after he swerved to avoid a drunk motorcyclist, and their car hit a tree.
She told the court he had said: "The last thing I remember before I was unconscious is the sound of her screaming."
He also told her Ms Morris was three months pregnant when she died.
The trial continues.