Since his father's death two decades ago, French billionaire art dealer Guy Wildenstein has faced a familiar foe at almost every turn.

Lawyer Claude Dumont-Beghi has doggedly pursued him over accusations he cheated his stepmother Sylvia out of a fortune from the inheritance. The dispute sparked a criminal probe in which Wildenstein was accused of concealing from French tax authorities paintings worth hundreds of millions of dollars in offshore trusts.

As it turns out, Dumont-Beghi had a little secret of her own. The longtime attorney for the late Sylvia Wildenstein hid about 4.5 million euros ($5.1 million) she got from her client in the mid-2000s in an undeclared account at HSBC Holdings Plc in New York. But French authorities eventually found out, leading to her conviction for tax fraud and money laundering in a case where she was criticized for using a setup in the British Virgin Islands to conceal the assets.

The yet-unreported guilty verdict emerged as part of a top court attempt by Dumont-Beghi to challenge her conviction that was only partly successful. Cour de Cassation judges in Paris suggested this month her sentence may need to be lightened. Dumont-Beghi admits some passivity as she was facing a personal drama but denies any intent to conceal assets. She says she has paid tax authorities what she owed.

The conviction adds an ironic twist to the Wildenstein saga as the family's most vociferous critic stands accused of something she has denounced all along: tax evasion.

For years, Dumont-Beghi publicly excoriated the Wildenstein family for dispossessing the second wife of Guy's father, Daniel. Tax fraud, she told a group of senators shortly after publishing her first book on the Wildensteins in 2012, "dooms" the economy if it's not tackled.

Dumont-Beghi's campaign culminated in a widely watched Paris criminal trial against Guy in 2016, with the proceedings focused on the offshore trusts set up by his father Daniel to stash artworks worth more than $1 billion. The Wildensteins have maintained that the art wasn't legally Daniel's, but belonged to the family trusts and shouldn't count for estate taxes.

Over weeks of hearings, prosecutors accused Guy of misusing trusts and transforming them into "piggy banks," and sought a 250 million-euro fine and a prison sentence. Guy was spectacularly cleared in early 2017. The acquittal was challenged by the Parquet National Financier but the art dealer won again on appeal.

The final outcome of the case remains uncertain after France's top court ordered its reexamination earlier this year and hinted that the path toward a victory for Wildenstein would be much narrower. Separately, the Wildensteins lost the first round of their civil battle against tax officials worth hundreds of millions of dollars.

Turn the clock back to Daniel's death two decades ago to understand how Dumont-Beghi entered the picture. When it came time to settle their father's estate, Guy and his brother claimed he had a net worth of 40.9 million euros and offered to cover the bill by giving a set of bas-reliefs by Marie Antoinette's favorite sculptor. Unaware of the existence of the trusts back in 2002, French tax officials accepted.

The matter seemed settled until Sylvia hired Dumont-Beghi a few years later and turned against the family. Daniel's second wife sued, claiming her stepsons had told her the taxes would bankrupt her if she didn't relinquish her estate rights. She won a key round in 2005. French tax authorities took notice, as did criminal investigators, launching Guy's legal woes.

Meanwhile, far away from the public eye, the Dumont-Beghi tax case has also been a roller-coaster ride that has now reached France's top court. Tax officials say she skipped nearly 150,000 euros in income tax and a little over 120,000 euros in wealth tax. They also brought in the criminal authorities, who built a court case against her.

A Paris criminal court initially found Dumont-Beghi guilty in February 2019 of aggravated tax fraud and money laundering. Her 18-month suspended sentence was confirmed on appeal a year later but she got a heavier fine: 750,000 euros, up from 100,000 euros.

The case then went to the Cour de Cassation, which maintained the money laundering conviction but questioned the aggravated circumstance applied to the tax fraud accusations by appellate judges. The top court pointed out in a Dec. 1 ruling released this week that such aggravated circumstances — having an account or an interposed structure abroad — were specified in France law after the fact.

The ruling suggests Dumont-Beghi will only have an opportunity to argue no aggravated circumstance can be found against her. The top court, however, backed several elements against her, stating that her offshore company was "an artificial setup intended to circumvent the application of French tax law," and that its "sole purpose" was to create a veil of opacity and conceal her undeclared account in the U.S.

Dumont-Beghi says her lawyer considers the lower court, which will reexamine the case, has full latitude to decide whether the tax fraud accusations should stand.

For now, Dumont-Beghi finds herself in the same boat as her foe Guy Wildenstein: on the wrong side of a civil and criminal battle with French tax enforcers.

Gaspard Sebag reports for Bloomberg News.

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