Roger said the celebrity socialite has agreed to plead guilty Monday in Las Vegas Justice Court to two misdemeanors stemming from the Aug. 27 arrest.
Hilton, 29, was arrested inside the Wynn resort, after police say a small plastic bag containing 0.8 grams of cocaine fell out of her Chanel purse as she reached for a tube of lip balm in front of a police lieutenant.
A plea agreement obtained by the Las Vegas Review-Journal shows Hilton will plead guilty to drug possession and obstructing an officer.
Hilton will serve a year of probation, the newspaper reported. She also must complete a drug abuse program, pay a $2,000 fine and serve 200 hours of community service.
Roger and defense attorney David Chesnoff confirmed the details of the plea deal reported by the Review-Journal. The original felony cocaine possession charge would not have resulted in any jail time.
Roger said he wanted to obtain a pledge from the hotel heiress to stay out of trouble, and said the plea deal accomplished that goal.
"If she is arrested for anything besides a minor traffic violation she will spend a year in jail," Roger said. "There will be no discussion. The court will have no discretion."
Chesnoff said the agreement does not call for Hilton to be assigned a probation officer, meaning she can't break any laws but won't have anyone to report to or check in with.
"Ms. Hilton understands the seriousness of the situation and appreciates the chance she is being given," Chesnoff told The Associated Press. "She has to stay out of trouble."
Hilton initially told police the purse and cocaine were not hers, but claimed some items in the bag, including rolling papers, $1,300 in cash and several credit cards.
She was questioned by police after her boyfriend, Las Vegas nightclub mogul Cy Waits, 34, failed field sobriety tests given by a motorcycle officer.
The couple was stopped in a black Cadillac Escalade after the officer smelled a "vapor trail" of marijuana smoke.
Hilton was banned from two Wynn resorts on the Las Vegas Strip after the arrest, and her boyfriend was dismissed as a nightclub partner.
Hilton briefly faced a marijuana charge in July after a FIFA World Cup match in South Africa, but the case was dropped when a woman who was with her pleaded guilty to possessing the drug.
Hilton is scheduled to appear at 7:30 a.m. Monday before Judge Joe M. Bonaventure, Chesnoff said.
Associated Press Writer Ken Ritter contributed to this report.