Gina Rinehart tried to get swimmer Grant Hackett to convince her eldest daughter Bianca to pull out of the family's long-running court battle.
In an email sent to the swimmer late on Sunday, Mrs Rinehart said: "(I) suggest you advise her urgently please, no money is worth this, she should withdraw her nomination as trustee, rather than be found a liar in the witness box."
The plea to Hackett was made just 36 hours before Bianca's scheduled appearance in the witness box yesterday - the first member of the family to give evidence in the near three-year stoush over the family's multibillion-dollar trust.
But Hackett didn't reply to the mining magnate and instead forwarded the email to Bianca.
"Just got this from your mum . . . good luck, you've done the hard yards and you're obviously in a great position," he wrote.
The move by Mrs Rinehart to recruit her daughter's former boyfriend failed to deter Bianca from taking the stand in the NSW Supreme Court yesterday.
Both sides of the family are stuck in an acrimonious deadlock over future leadership of the trust, which owns almost a quarter of family company Hancock Prospecting, and 37-year-old Bianca is the only family member to have put her name forward as a potential trustee.
But Hancock Prospecting and the mining magnate's youngest daughter and heir apparent Ginia are fighting her appointment, which is backed by her brother John Hancock.
In a series of exchanges with Ginia's lawyer Richard McHugh yesterday, Bianca said that a lot of what the court was told about her and her brother was propaganda "so I'm here to enlighten the court with the truth".
She defended John over allegations he had shown contempt for Ginia and said his comments that their youngest sibling was an "oxygen thief" and "intellectually disabled" needed to be taken in context.
"He is challenging my mother and her apparent desire to leave the family company in the hands of somebody who, from the outset of this case, had asked to be rendered useless because she's so lost in it and now it would appear she is spearheading it," she said.
"So, if she is so easily influenced, it would appear, to change her position on things, I understand my brother's frustration and that's all he is trying to do, is vent his frustration in that occurrence."
She also said comments she had made to her mother that John was "beyond our help" and that his "self-harming behaviour has developed into a hate-driven machine which is now been directed towards you (Mrs Rinehart) had been made to appease the mining magnate".
Asked if she expected a "high level of conflict" in the family in future, she said: "It depends how Your Honour decides this current proceeding, I would imagine."
Mr McHugh: You are still worried about personal attacks being made on you, are you not?
Bianca: I would assume they will probably continue but I will withstand them.
Mrs Rinehart's move to get Hackett to intervene mirrors a similar overture made on behalf of the mining magnate by Federal Agriculture Minister Barnaby Joyce at the start of the long-running legal battle.
In September 2011, the then Nationals Senate leader emailed 26-year-old Hope Welker - Mrs Rinehart's second-youngest daughter whom he had never met - urging her to drop the case.
"All good families have their problems but before it gets really out of hand, I would try to get it back in house," he said.
Hope later withdrew from the battle and got $54m from her mother on condition she did not criticise her mother.
The court has been told that Ginia Rinehart - the only one of Mrs Rinehart's children to have sided with her throughout the case - is bound by a similar agreement.
The case continues.