One of two men charged in the beating death of an 85-year-old woman in her Waialae Nui home has made a deal with the prosecutor to testify against the other defendant.

Zachary T. Robinson, 20, pleaded guilty in Circuit Court yesterday morning to reduced charges of manslaughter and first-degree assault in the Jan. 21, 2010, beatings of the elderly woman and her 40-year-old caregiver.

Mary Lee Koskinen died of her injuries two weeks after the attack. The male caregiver suffered critical injuries.

Robinson also pleaded guilty to first-degree robbery in connection with the case and first-degree burglary in a separate case. He was facing second-degree murder and first- and second-degree attempted murder charges.

As part of his plea deal, Robinson has agreed to accept a 30-year prison term when a judge sentences him in August and to testify against his co-defendant, 22-year-old Cody Mikami, whose trial is scheduled for July.