Hours later, Mr Waipouri was found viciously beaten to death. His friend "Ash" would be one of two men charged with his murder.
Arnopp would later admit to killing his mate, to prison staff and police. On Monday, Arnopp changed his plea to guilty at the High Court in Palmerston North.
The other man charged with murder was Arnopp's friend Andre Gilling, who has maintained his innocence.
Both men accused consumed a toxic mix of cannabis, party pills - the "strongest you could buy" - and alcohol before the attack.
Exceedingly loud, obscene music, heard in a flat below, masked the depravity upstairs - Mr Waipouri's body was found bloodied and savagely beaten on the lounge floor. His penis and ear both severed.
Blood - soaked, splattered and smeared - covered most surfaces, including the ceiling, of the small flat.
Yesterday, a jury of seven women and five men deliberated for nearly 11 hours to consider Gilling's guilt of either murder or manslaughter.
The result was a hung jury.
The Crown sought a retrial and Gilling was remanded in custody until February 5 when a trial date would be set.
During the eight-day trial, Gilling said he'd had no involvement in the attack, except to shake Mr Waipouri, urging him to get up. He'd witnessed Arnopp kicking Mr Waipouri.
It would never be known whether a single blow or multiple blows killed Mr Waipouri, the court heard in evidence.
The Crown had argued that both men had equal involvement in Mr Waipouri's death. Blood, in equal measure, was found on both the accused men's clothing, skin and shoes.
A "fresh" abrasion on Gilling's instep was also found to contain blood from Mr Waipouri.
Outside court, Mr Waipouri's family members were stunned at the jury's result. "Now we've got to go through this stuff again. And look at him [Gilling] - the man a part of my brother's murder - again," Joan Waipouri said.
When police described by phone the extent of her brother's injuries, Ms Waipouri said she envisaged two "ghouls" feeding off her brother's bloodied body. But somehow, she said, the two young men accused didn't fit this vision in court.
"I thought, 'Oh God, they're young . . . and human'."