Justice Helen Winkelmann's decision to grant the teen a discharge without conviction prompted an angry outburst from the Dudleys in court yesterday.
"I don't understand," Mrs Dudley said through tears from their Kelston home last night.
"An attack was made, he pleaded guilty to it. How can it slip through our justice system? A life was taken.
"We have been deprived of justice."
The couple, who have four other children aged from 11 to 20, would check with "legal advisers" about what to do next, including whether an appeal was possible.
The incident which led to Stephen's death, began with an argument between he and a 16-year-old. The confrontation attracted the older boy.
After he and the 16-year-old launched a sustained attack, the pair left Stephen on the ground unconscious.
Stephen was rushed to Auckland City Hospital but died a short time later.
Critically, medical examinations showed an undiagnosed heart condition contributed to his death.
As a result the Crown withdrew the manslaughter charge against the older defendant after receiving two expert reports on cardiac pathology and arrhythmia.
The younger boy also admitting assaulting Stephen and was discharged without conviction.
After yesterday's proceedings, Mr and Mrs Dudley sat at home, beneath the photo of Stephen in their living room, to reflect on what happened.
"What message is being sent out there - what are we telling our youths, that it's okay to do harm and take someone's life?" Mrs Dudley said.
"It was a brutal, cowardly, thuggery act. Our son did not deserve one second of his last moments ..."
Mr Dudley, whose anger boiled over in court, said he did not regret his outburst from the public gallery.
"You're f***ing joking," he said from the public gallery.
"His actions caused my son's death . . . That's justice for you, New Zealand. The law's an ass."
Lawyer John Munro, who was representing the 18-year-old, said his client had been excluded from school and experienced severe social isolation because of what happened.
He called it "a very, very poor error of judgment" and stressed how much he had learned from the experience, giving speeches to young people at his church about the consequences.
Justice Winkelmann said there was no indication he had ever resorted to violence in the past and he could not be sentenced on the basis that his actions caused Stephen's death.
"You have lived and will live your life in the knowledge of your role in the events that ended with Stephen's death and I am satisfied that you do feel the weight of that," she said.
"I consider that there is a real and appreciable risk that your transition into adulthood, given your current prospects and educational ambitions, will be significantly prejudiced should a conviction for violence be entered against you."
Read a recent interview with Stephen Dudley's parents