Le Pen told journalists on Wednesday that Griset had "proved" that she had fulfilled the conditions of her contract as an accredited assistant, which stipulated that she had to live in Brussels and work in the European parliament.
Griset, who is a longtime friend of Le Pen's, has shown that she "stayed with another parliamentary assistant, that she swiped a badge, that she was registered with the tax services", Le Pen said, adding that her assistant worked both in Brussels and in France "when I decided, because I'm the one who decides where to carry on the fight against the European Union".
Investigators concluded that Griset in fact worked full-time for the party, with her salary being paid by the European parliament and have proceeded to reclaim the money.
Sleeping on the couch
After the UK's Brexit vote Marine Le Pen called for France to leave the euro, a demand that features in her presidential campaign programme.
This week she confirmed that the party also wants a referendum on leaving the European Union itself, althoug in December she said she might vote no in it if the EU had "totally changed".
A petition for the "end of the European Union", described as "unreformable", now features on the party's website.
Asked why she had no address in Brussels, Griset told European fraud office (Olaf) investigators that she "stayed with friends who had a big house".
At first she had her own room, she said in testimony which has been leaked to l'Obs magazine, but work on the house meant that she later had to sleep on a fold-out sofabed.
An anonymous witness told Olaf that Griset had "never spent an entire week in Brussels".
The investigators were also surprised to find that her daughter continued to go to school in France.
"She didn't want to change school," Griset told them, adding that she had left the girl in the care of her mother and her partner.
The inquiry also only found evidence of her presence at the parliament for a total of three hours between 1 October and 31 December 2014.
Griset claimed that she went there more frequently, entering the building via the carpark in Le Pen's company, although assistants are supposed to go in by a door specifically assigned to them and are strictly forbidden to use the MPs' door, according to l'Obs.
Pay as you earn
The parliament, which stresses that the procedure is administrative and not a punishment, is to start deducting the 298,400 paid to Griset from Le Pen's February paycheck, which is due on the 15th of the month.
The sum will be repossessed by taking 50 percent of her salary, 100 percent of her expenses and 50 percent of her attendance allowance, to a total of about 8,000 euros a month, leaving the Eurosceptic MEP with 3,000 euros a month take-home pay.
Another recovery is to be enacted for the 41,500 received by Le Pen's bodyguard, Thierry Légier, whom she also put on the European parliament's payroll.
Le Pen claims political persecution
The far-right leader vowed to fight the move and says she has sued top officials of Olaf and the European parliament.
"I will not give in to persecution, to this unilateral decision taken by political opponents provisionally enforced in violation of the law, of the right to defence, without proof and without waiting for the courts, to whom I have appealed, to rule on the case," she told the Reuters agency.
Although Le Pen argues that she has not replied to the charges, sources close to the inquiry insist that she and her representatives have had several opportunities to do so.
The head of the mainstream right group in the parliament, Manfred Weber, called her accusations "scandalous and unfounded".
"Marine Le Pen seems to consider herself above the rules," he commented. "That shows the National Front's true nature."
Over 1.1 million euros sought from FN MEPs
The European parliament is seeking recovery of a total of 1.1 million from six MEPs elected on the National Front's ticket - Jean-Marie Le Pen, who has since fallen out with his daughter and been suspended from the party, Bruno Gollnisch, Mylène Troszczynski, Sophie Montel and Dominique Bilde - sources have told l'Obs.
A further 20 allegedly dodgy contracts are also under investigation.
In December French judges opened an investigation into fraud charges against the MEPs.
Le Pen's lawyer, Marcel Ceccaldi, claims that many other MEPs' assistants actually work for their parties, citing 13 members of Spain's Socialist Party (Psoe) and 14 Polish MEPs from "conservative and reformist" parties.
While he says that is proof of persecution, officials say that 96 repayment procedures were enacted in 2015 and that it shows that the procedure is not politically motivated at all.