Friends said Rosillo was speaking to a former Macleans College student when the girl's boyfriend - believed to be a former Mt Albert Grammar student - turned up.
"The guy was yelling at the girl. Warren went down to try and defuse the situation," Meagher said.
Rosillo, who worked for Diesel Rescue in East Tamaki, was meant to be going to a work shout at the Thirsty Whale bar in Three Kings on Friday night.
Instead, he went to a 21st party at Ye Olde Bailey Pub in nearby Pakuranga, but left after the bar staff refused to serve the group any more drinks.
They moved on to the birthday party on Greenhill Cres where about 20 teenagers were drinking at a house.
Ethan White said the tight-knit group of friends were devastated Rosillo had died.
"Why would you bring a knife to a fight over an 18-year-old relationship? We heard the yelling from the house and ran outside. Warren was lying flat on his back and Dillon was yelling 'Don't give up, you can't die'. I thought he had just fallen over drunk," White said.
The group was taken to the police station where they were separated and questioned for more than four hours. "It was a long night," White said. "They needed every single detail."
Dale Clubley, who lives across the road, said he was woken up about 11pm by loud shouting and swearing.
"I walked up and saw a group of youths running towards the Pakuranga Highway and a group of kids kneeling around the body. I asked them if anyone had called 111 and didn't really get an answer. The kids were on the verge of hysteria. It was pretty intense," Clubley said.
"My partner's nephew saw all of it from his bedroom window. A scrap broke out and then broke up, then one of the guys come running back. It was just stab, stab, stab and he went running off."
Detective Senior Sergeant Dave Glossop said police had one suspect for the killing - but had yet to make an arrest. He said several knives had been found in the area of the murder.
The Herald on Sunday knows the identity of the man but have not printed it after a request from Glossop.
Rosillo moved to New Zealand from the Philippines at age 13. He started high school on the North Shore before his family moved to Pakuranga three years ago.
The former Edgewater College student was nicknamed "WOF" because he was so keen on cars.
Close friend Marissa Daniels said: "He was the Warren of Fitness. You wouldn't go and buy a car or do anything to your car without Warren there."
The two had dated in high school and remained good friends. Marissa's family had taken Rosillo in when his relationship with his parents became strained.
Friend Eiken Elazegui said Rosillo had begun to patch up his relationship with his parents. Elazegui said he visited Rosillo's parents yesterday, who were in shock.
"It's really hard for them. They don't believe it," Elazegui said. "He hadn't talked to his parents for a year [then] a month ago they started talking again."
Meanwhile, the unexplained death of a 32-year-old man found lying in a Helensville garden on Friday is not a homicide, said a police spokesman.
