Marc Labelle asked Quebec court Judge Carol St-Cyr to reduce the amount of the reimbursement and to give Thibault, 76, a sentence to be served in the community.
She pleaded guilty to the two charges last December. Fraud carries a maximum prison sentence of 14 years, while the maximum for breach of trust is five years.
The wheelchair-bound Thibault, who held the provincial vice-regal post between 1997 and 2007, was charged after a joint report filed by former provincial auditor general Renaud Lachance and his federal counterpart at the time, Sheila Fraser.
The document suggested more than $700,000 in alleged improper expenses had been claimed. Labelle said in December that Thibault acknowledged she unjustly claimed $310,000, while the Crown estimates the amount at $430,000.
As sentencing arguments began Friday, Labelle called Thibault's partner, Real Cloutier, to the witness stand.
Cloutier, who was Thibault's longtime aide-de-camp, described himself as her "best friend, chauffeur, bodyguard, cook and assistant gardener."
He said he examined her accounts in detail and that the money to be refunded should actually be $250,000.
Cloutier alleged that the $4,800 monthly sum she was meant to receive from the Quebec government ended up, between February 1997 and May 1998, in an account that belonged to her office and not in her own account.
He also testified that Thibault is $800,000 in debt and that she frequently used her own cash while on official duty when her expenses outstripped the money she received from the federal government.
"The line of credit was Mme Thibault," said Cloutier, who accused some people of "crucifying Lise Thibault in public."
Cloutier broke down in tears as he told the court that Thibault prefers to stay home because of the reaction when she goes out in public.
"At the grocery store, people would insult her," he said. "When she took money out of the bank machine in Saint-Jerome, if there were seven people in the lineup, they called her a thief."
Labelle said it was important for Cloutier to testify so St-Cyr could get a better idea of how much money should be paid back.
''Jurisprudence says a judge, before imposing a reimbursement, has to be in a situation where the amount of the reimbursement is easily determined," he told reporters at the end of the day's proceedings.
"And we asked Mr. Cloutier to produce all the documentation and to describe the work he has done, to convince the judge it is not an amount that can be easily determined.''
Thibault's trial heard the money was allegedly spent on gifts, trips, parties, meals and skiing and golf lessons.
She switched pleas last December because, according to Labelle, she came to a better understanding of the evidence and the law.
That was Thibault's first appearance in court since August when she suffered an epileptic seizure that interrupted her cross-examination.
She testified she had little to show financially for her time as vice-regal — a divorce ate into her savings and she said she now lives on a $30,000 pension.
Last summer, St-Cyr ruled against a pair of motions filed by Labelle, who argued the case should be dismissed because the accused benefited from royal immunity.
Labelle said that meant Thibault was not a civil servant and therefore could not face criminal charges.
The judge wrote that, according to constitutional law, the lieutenant-governor does not enjoy the same benefits as the Queen.
St-Cyr also noted that under the Constitution, the lieutenant-governor is a civil servant, adding such an affirmation is even posted on the lieutenant-governor's website.
The sentencing arguments will resume May 21.