Heriberto Viramontes denies the attempted murder of Silverbridge woman Natasha McShane and her friend Stacy Jurick in Chicago in April 2010.
The women were badly beaten in a vicious baseball bat attack under a viaduct.
Marcy Cruz took to the stand to testify against Viramontes, her former boyfriend. Cruz, who is 28, has already pleaded guilty to her role in the attack which left Natasha McShane brain damaged and unable to walk or talk.
She's already been sentenced to 11 years for each of two counts of attempted murder.
Cruz told the court how she and Heriberto Viramontes were driving around Bucktown that night when they spotted the two women.
She said Viramontes told her he didn't want her "involved in nothing I'm about to do" before grabbing the bat and jumping out of the van.
Cruz said Viramontes got back into the van with two purses before telling her to drive off.
When cross-examined by defence lawyers, they said that Cruz's story had changed from when she first spoke with police and prosecutors.
They claim another man is responsible for the attack and said that Cruz implicated her former boyfriend because she will say anything as long as it benefits her.
Speaking outside the court room Marcy Cruz's father said the family feel sorry for what happened to Natasha McShane and Stacy Jurick.
Natasha, who was a 23-year-old exchange student at the University of Illinois at the time of the beating, has been left severely injured. She she has now returned home to Northern Ireland but struggles to walk and still cannot talk.
The trial at Cook County Criminal Court in Chicago continues.