The Court of Appeal has ruled that Mrs Lejonvarn owed her former friends a 'duty of care'
Basia Lejonvarn, 52, insisted it was not "fair, just or reasonable" to expect her to compensate Peter and Lynn Burgess.
The work she carried out on the garden of the couple's £5 million home in Highgate, north London was "gratuitious", top judges were told.
But now the Court of Appeal has ruled that Mrs Lejonvarn owed her former friends a "duty of care" to carry out the work competently.
The ruling leaves Mrs Lejonvarn facing huge legal costs bills and opens the way for the Burgesses to sue her for £265,000 damages.
Lord Justice Hamblen said the relationship between them was not just "informal or social" and "the context was a professional one".
The row between the former friends broke out when the Burgesses decided they wanted to iron out the steep slopes in their garden in 2013.
They wanted restful terraces, paths, lawns and mood-lit flower beds but were not happy with a £150,000 quote from an established landscape gardener.
Mrs Lejonvarn offered to help and she found Polish contractors to carry out the work at a lower price.
But, within months, she and recruitment tycoon Mr Burgess, 58, were engaged in an acrimonious "email war".
He and his wife blamed Mrs Lejonvarn for cost-overruns and a litany of defects allegedly left behind.
The builders were ordered off the job and the project was eventually completed by the gardener whose quote the couple had earlier rejected.
Mrs Lejonvarn and Mr Burgess engaged in an acrimonious 'email war'
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The Burgesses say the transformation of their garden cost them £265,000 more than it would have done had Mrs Lejonvarn never been involved.
She protested that she had merely provided free advice to the couple on a friendly basis, in a "non-commercial, informal and social context".
Judge Alexander Nissen QC, in January last year, agreed that there was no contract between Mrs Lejonvarn and the Burgesses.
But he broke new legal ground when he ruled that she nevertheless owed them a professional duty of care.
Although she was not initially paid for her work, Mrs Lejonvarn had "assumed responsibility" for the garden project, he said.
She had provided her services to the couple "in a professional context and on a professional footing".
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And that fact that her services were "provided gratuitously did not mean that they were informal or social in context".
This was not brief ad hoc advice but was a significant project which was being approached in a professional way
Lord Justice Hamblen
Rejecting her challenge to that decision, Lord Justice Hamblen said: "This is a case which concerned Mrs Lejonvarn voluntary tendering skilled professional services in circumstances where she knew the Burgesses would rely on the proper performance of those services.
"This was not brief ad hoc advice but was a significant project which was being approached in a professional way."
The judge, sitting with Lady Justice Gloster and Lord Justice Irwin, added: "Although she was not going to be paid initially, the expectation was that she would be paid for later work.
"She held herself out as having professional skills. She said she would perform professional services and did so.
"She was aware that the Burgesses would be relying upon her to properly perform those services and it was foreseeable that economic loss would be caused to them if she did not."
The judge ruled that Mrs Lejonvarn's duty of care to use reasonable care and skill extended to designing and project managing the garden transformation and controlling its cost.