"If he is called, he will come," said Samantha, 55.
Markle is suing Associated Newspapers for publishing a private letter -- reportedly written to her father -- in the newspaper, the Mail on Sunday.
The civil lawsuit accuses the newspaper of copyright infringement, misuse of private information and violating the U.K.'s data protection law with its publication of the letter.
Documents filed at the High Court show that the newspaper plans to rely on evidence from Thomas, stating that he “had a weighty right to tell his version of what had happened between himself and his daughter, including the contents of the letter.”
Papers drawn up by lawyers for the newspaper argue that members of Britain's royal family "generate and rely on publicity about themselves and their lives in order to maintain the privileged positions they hold and to promote themselves."
The paper also argues that the letter's publication was in response to a "one-sided" article in People magazine in February 2019 that featured an interview with five unnamed "close friends" of the duchess, which referenced the letter -- meaning its existence was in the public domain.
The documents came to light this week amid the firestorm of attention that followed Meghan and Prince Harry's decision to issue a statement announcing that they wanted to step back from their royal roles, become financially independent and split their time between Britain and North America.
Queen Elizabeth II convened a family summit on Monday at her Sandringham estate in eastern England and decided the couple could live part-time in Canada.
Meghan was seen in Canada for the first time since the crisis began when she visited the Downtown Eastside Women's Centre in Vancouver to discuss issues affecting women in one of the city's poorest neighborhoods. The shelter posted a photo of the duchess' visit to Facebook.
Fox News' Melissa Roberto and The Associated Press contributed to this report.