Their rugby coach had previously told them the team would be disbanded if anyone was caught fighting, he explained.
Stephen had almost left the area when he was called back by another teenager, and the pair met in the middle of the rugby field, surrounded by others.
The boys squared off but other than swearing and posturing nothing else happened, the witness said.
"Everything was verbal until [an older boy] made his way over," the teenager told the court.
"He was just heated and we were trying to stop him."
But he said the bigger boy was too strong and ran at Stephen from the side.
The witness said he hit the younger boy with a "clothes-line wrestling move, slamming Stephen in his neck and knocking him straight to the ground".
Others who gave evidence today described the blow differently - one calling it a punch, the other a shoulder charge to the jaw.
Both boys set upon the victim, who ended up on the floor covering his head to shield himself from the blows.
The teenager told the court he knew something was wrong when Stephen collapsed.
"He was knocked out and had froth coming out of his mouth," he said.
"I saw lots of veins bulging out of his neck. They were strange veins that you don't normally see."
The next witness told the court there was a frantic discussion between the boys over whether to call an ambulance and if they should delete videos of the fight they had recorded on their phones.
The teenager said something "snapped" inside him and he put Stephen in the recovery position.
"He was on the ground making weird noises, like a moaning sound. It looked like he was going into a seizure. His eyes were going white and he blacked out," he said.
The witness said he gave Stephen CPR until paramedics arrived because the others were too scared to do it.
Another rugby teammate of Stephen described the older boy's forearm blow as a "cheap shot".
As the teen lay on the ground, the two who assaulted him wandered away.
"They seemed proud of what they had done," the witness said.
"They said to the rest of us: "is there anyone else?" but nobody had the guts."
When asked about the size difference between Stephen and the bigger boy, he told the court there was probably a 20kg disparity, comparing it to diminutive All Black Damian McKenzie fighting Ma'a Nonu.
"Stephen was keen to play first XV but he won't have the chance now," he told the court.
Stephen died in hospital shortly afterwards and two boys were arrested and charged.
One of them, 18 at the time and whose name is permanently suppressed, was discharged without conviction at the High Court in Auckland in 2014 after a manslaughter charge was dropped to one of assault with intent to injure.
His co-defendant - 16-years-old at the time - also admitted assaulting Stephen and was similarly discharged without conviction three months earlier.
Medical examinations showed an undiagnosed heart condition contributed to Stephen's death, which was critical to the prosecution dropping the manslaughter charge.
The victim's parents were devastated by the court's decision to release the teenagers without consequence.
"He was a lot smaller than you and you attacked our son from behind. He had no idea about the attack he was to be subjected to," father Brent Dudley said at the sentencing hearing.
"Any thoughts of forgiveness are out of the question at this stage. I hold you entirely responsible for the death of our son ... you own that."
The enquiry before Coroner Gordon Matenga is scheduled to last all week.
