SALEM – A former Peabody woman admitted she helped rob a man of $400 after he gave her a ride home from a downtown bar in Peabody two years ago.Dawn Coleman, also known as Dawn Sullivan, 37, formerly of 4 Mill St., Apt. 5, Peabody, pleaded guilty Wednesday afternoon in Salem Superior Court to a charge of armed robbery before Judge Howard J. Whitehead.The judge postponed sentencing her until June 2 because the prosecutor, Greg A. Friedholm, was in court in Lawrence on another case and he should be given the opportunity to allocate what he feels her punishment should be, given Sullivan has a prior conviction of armed robbery.Whitehead, however, did inform Coleman, who was very tearful throughout the plea, he was probably going to sentence her to two and one-half to four years in prison.The charge concerns an incident that began in the early morning hours of Jan. 15, 2006, when she asked a man she met in a downtown Peabody bar to give her a ride home in his BMW to Mill Street where she directed him to a parking lot.An unknown man came up to the car, held an 8-inch cutting knife to the victim’s throat while she shut off the car engine and took his wallet, containing $400 in cash, credit cards and identification.The two then ran off, but the victim was able to provide police the name of his female attacker, who police were familiar with.When police spoke to Coleman she conveyed her guilt and said “I knew we shouldn’t have done what we did,” said Assistant District Attorney Michael A. Sheehan who was standing in for Friedholm, as he quoted from a police report.Coleman, who had been free on bail, was expected to plead her case out last month, but when she heard what the judge was going to impose for a punishment she decided to go to trial instead, but before Whitehead could send her case to another judge for trial that day, she took off and was arrested last week on the warrant.