Skeen began swinging at Tipene with the shard of glass as the victim backed off with his hands up, attempting to defuse the situation, she told the court.
"Then the bottle went into Luke's throat."
Togiavalu and her cousin retreated to a nearby roadside as the melee subsided.
"I was a little bit in front of him and I turned around and he dropped his hands and all the blood started squirting out of his neck," she said.
"He was whispering the word 'ambulance' and I started screaming for help . . . I was just sitting there holding him, trying to stop the blood."
Tipene died in hospital four hours later.
Defence counsel Lorraine Smith said her client - who was 16 at the time - may be guilty of manslaughter but lacked the requisite intent to be convicted of the more serious charge.
"The Crown will have to prove beyond reasonable doubt that Vincent Skeen . . . swinging out with a broken bottle, turned his mind to the fact that what he was about to do might well kill and he decided to take the risk regardless. The defence says that's simply not the case," she said.
"Vincent would have been hard pressed to hit Luke in the neck on purpose. He simply swung at a boy who was older, bigger and stronger."
Neither Skeen nor Tipene had attended the party that night but they came together when a fight between their respective friends erupted on Great North Rd, Crown prosecutor Brian Dickey said.
"Mr Skeen stepped into what was supposed to be a one-on-one fight with Luke's friend and another young man," he told the jury.
"And at that point matters escalated as Luke, who had been standing by, stepped into the fight and hit Mr Skeen to the side of his head to assist his friend who at this stage had become outnumbered," the prosecutor said.
Dickey said Skeen became "enraged" and sought retribution, allegedly finding a beer bottle in the back of a ute and smashing it to form a "protruding, jagged blade".
There were at least seven blows with the weapon, according to the Crown, one of which caused a 12cm wound, which stopped just short of Tipene's spine and pierced his jugular vein.
"The question for you in this trial is likely to be: what was the intention or what was the defendant thinking?"
Smith accepted her client did something "very foolish" but could not have appreciated the consequences of his actions.
"If the blow had hit Luke in the chest or his shoulder or arm or just about any other part of his body other than his neck, we wouldn't be here," she said.
The trial before Justice Mary Peters is scheduled to last two weeks.
