LONDON — Johnny Hallyday, known as the French Elvis, built his six-decade show business career on old-fashioned rock ’n’ roll, but he also kept up with the times technologically. From 2012, his Instagram account shared a canny mixture of the personal and the professional, promoting his tours, his albums and his image as one of France’s most enduring stars.
And on Tuesday, it helped two of his children defeat his widow in the first stage of a legal battle over an inheritance that the French news media values at tens of millions of dollars.
When Mr. Hallyday died in 2017, two testaments were found in a safe deposit box. One of the documents, written in Los Angeles, appointed his wife, Laeticia, as sole heir and manager of his estate.
That will entirely excluded Mr. Hallyday’s grown children from two previous relationships, David Hallyday and Laura Smet, something not permitted under French law. The two have been fighting to prove that their father lived mostly in France and not in the United States.
Laeticia Hallyday, the singer’s fourth wife, told a court in Paris that he had settled in Los Angeles in 2007, his daughters Jade and Joy went to school there, and he had received a green card in 2014. She also, according to court documents, cited the fascination for Elvis Presley and American culture that was at the center of most of Mr. Hallyday’s life and work.
David Hallyday, however, offered something even more compelling: a chart of Johnny and Laeticia Hallyday’s locations from 2012 to 2017 as revealed by pictures from their public Instagram accounts.
The chart showed that Mr. Hallyday spent at least 151 days in France in 2015 and 168 the year after. He then spent eight months without interruption in the country, mainly because of his illness, before his death in 2017.
The court accepted the children’s argument, ruling that it had competence to decide on Mr. Hallyday’s estate.
The battle over the Hallyday inheritance, including performers’ rights on over 1,000 songs and properties in France, on the French Island of St. Barthélémy in the Caribbean and in California, has mesmerized France. Mr. Hallyday was an idol for many during his life and he received a hero’s tribute in Paris after his death, as 15 million watched on television.
But Tuesday’s decision also served as a reminder of the rising prominence of data from social media as evidence. Other than cases in which content is directly at fault, like bullying or harassment, social media posts have been used in divorce and criminal proceedings. In 2017, the International Criminal Court in The Hague issued its first arrest warrant relying on videos posted on social media as evidence.
“Everyone is still finding their feet with this,” said Emma Irving, an assistant professor of public international law at Leiden University in the Netherlands in a phone interview on Wednesday. “There is a gradient of approaches out there,” she said, referring to courts.
The attitude of today’s courts is similar to their predecessors in the early 20th century, when the admissibility of conversations on the telephone, then a relatively new technology, was still contested, according to a copy of the Harvard Law Review from 1918.
But the problem with social media, Professor Irving said, “is that the technologies that we have to develop fakes are very sophisticated, making it very difficult to tell the difference between genuine material and fake material.”
“What we’re facing now is making sure that we’re not prejudicing the right to defense,” she added.
The court in Paris said it handled the Instagram posts as public data in the same way it had other materials, such as press clippings. It was up to the widow, the court’s ruling said, to provide evidence to the contrary.
Ms. Hallyday plans to appeal the ruling.