The soldier, identified as Ghaisse M, was among a group of prisoners, including American aid worker Peter Kassig, decapitated by Islamic State jihadists in November 2014.
One of the killers was identified from a grisly IS video of the execution as Maxime Hauchard, a 24-year-old Frenchman from Normandy, and an international warrant was issued for his arrest.
French authorities have opened a judicial enquiry into "murders linked to a terrorist act" over the executions.
On Friday, he and his wife gave evidence to a judge.
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"It is the first time that a Syrian family is a plaintiff in a case implicating a French jihadist who has gone to Syria," lawyer Fabrice Delinde said.
He said the soldier's parents "were able to talk about the particularly atrocious circumstances surrounding the death of their child."
After six months he was handed over to the Al-Qaeda-linked Al-Nusra Front.
"We paid a ransom to the Al-Nusra Front. But they tortured him and sold him to Daesh (IS) who decapitated him," he said.
Attempts by Ghaisse's parents to bring the case to court in France were initially rejected, but the Paris appeals court later ruled it was admissible.