Senator Mike Duffy has re-hired two staff members who were embroiled in his criminal trial for fraud, breach of trust and bribery.

Melanie Mercer, who worked in Duffy's office from his 2009 Senate appointment until the time of his suspension in 2013, and Mary McQuaid, Duffy's P.E.I.-based policy adviser, are back on the senator's payroll. McQuaid has been hired as a special assistant to Duffy and Mercer takes on the role of executive assistant.

Mercer was a Crown witness during Duffy's criminal trial and was forced to defend her record-keeping practices — including her move to have Duffy pre-sign travel expense claim forms. The RCMP alleged that this practice amounted to fraud. Senate Speaker George Furey testified that it was a "poor practice."

"He was very busy, very high in demand, so when these ladies [other Senate administrative assistants] suggested having these pre-signed forms at the ready, it seemed like a good idea at the time. I thought it was very practical," Mercer said of the forms.

Mercer conceded that she did not tell Senate administration about this practice — "I just kept it between us in the office" — but said there was no malicious intent. Duffy, too, defended the practice during his testimony. He said it was necessary to get his travel claims processed in a timely manner.

"We were constantly hectored: 'Claims must be in on time. If they're not filed within 60 days, you have to eat the cost yourself,'" he testified.

Justice Charles Vaillancourt ultimately batted away accusations of fraud, ruling that pre-signed travel claims were not an uncommon practice on Parliament Hill.

"I do not attach any sinister motive or design in this practice on the part of Senator Duffy," the judge said in his ruling. "Nor do I consider Senator Duffy's use of them a negative reflection on his integrity or credibility."

Senator Mike Duffy's office assistant, Melanie Mercer, testified at his criminal trail, and was asked about her practice of using pre-signed travel expense forms.

Mercer, a seasoned Hill staffer, began her career in politics working as a receptionist in then-leader of the opposition Stephen Harper's office. Later, she worked alongside Sandra Buckler, Carolyn Stewart-Olsen and Dimitri Soudas, all of whom served terms as Harper's director of communications. She also worked as former B.C. MP James Moore's parliamentary assistant and in former MP Gordon O'Connor's office as a media liaison.

She found herself without a job in 2008 and was scooped up by Duffy after his appointment to the Red Chamber. The two appear to have a warm relationship as evidenced by his demeanour toward her during her days in court.

Staffer helped redact diaries

One of Duffy's 31 criminal charges was related to expenses McQuaid filed for a trip she made to Ottawa. Mercer was out of the office on maternity leave and Duffy brought McQuaid to Ottawa to get her up to speed on the tasks she would be covering off in her colleague's absence.

Duffy had spent his office budget for the year — "There was no money left in the kitty," Duffy explained during his trial testimony — so he had his friend and contractor, Gerry Donohue, cover the costs with money he had given him earlier in the year to pay for other expenses. Donohue reimbursed McQuaid directly with a cheque for $1,068.08. The Crown tried to frame this payment as a kickback.

The court learned that McQuaid also helped Duffy to redact four years of his diaries during the height of expenses scandal. Duffy was preparing to send his electronic calendar to Nigel Wright, the prime minister's former chief of staff, to prove that he had spent a considerable amount of time on Prince Edward Island and that his primary residence was indeed at 10 Friendly Lane in Cavendish.

Duffy hoped the diaries could help him make his case to the PMO that he had done nothing wrong.

Those diaries were ultimately made public by the RCMP in advance of Duffy's criminal trial and they revealed a great deal of personal information about the senator, including his weight, eating habits, private meetings and his thoughts on key players in Ottawa.

Duffy fondly described McQuaid as a trusted staff member during his time in the witness box.

"She is warm and engaging and people felt good calling her," he said. "She did most of the work, for the people who needed help."

He said he kept her on in P.E.I. to be his eyes and ears on the island and to better provide services to the people of Charlottetown, who were represented by a Liberal MP.

Duffy was cleared of all charges in April.