Motorbike enthusiast George Kodogiorgos was drag racing along a Melbourne freeway when he hit a concrete barrier, flew into a steel signpost and fell eight metres to his death.
He and mate Martin Durkan had been weaving through traffic on their bikes at average speeds exceeding 150km/h following a friend's birthday dinner in July 2018.
Mr Kodogiorgos crashed while exiting the Tullamarine Freeway at 100km/h. He was found dead on a bush-covered embankment underneath.
Durkan pleaded guilty in Victoria's County Court on Wednesday to dangerous driving causing the 32-year-old's death.
He said he initially didn't realise what had happened to his riding buddy, and kept speeding to rejoin friends from their Melbourne Street Racers group at another restaurant.
Durkan then went home and discussed news reports of the fatal crash with his housemates before being arrested two days later.
Mr Kodogiorgos' mother, Penny, said her grief was unbearable.
"The last words I said to my son were 'George, be careful'," she said in a statement read by prosecutors.
"Without George, there's no life or future."
Her family urged others "to be kind and safe on the roads".
"We hope his loss will not be in vain."
Durkan waited on the side of the freeway for a minute or two after losing sight of Mr Kodogiorgos.
He then sped through a tunnel on a lane closed for roadworks at more than double the 60km/h speed limit, while performing a wheel stand.
"I'm responsible. We were doing silly s*** and he died because of it," the 51-year-old later told a psychologist.
"Every time I ride past that spot ... it's a constant reminder of my actions.
"I will bear this forever."
Durkan was described by friends and colleagues as a reliable, compassionate man with a strong sense of social justice and who had dedicated his life to science.
Judge Felicity Hampel noted an "extraordinary dissonance" between the man described and the one who raced along a freeway at "shockingly irresponsible" speeds for 15km.
"It's as if something happens when he gets on a bike," the judge said.
Lawyer Malcolm Thomas agreed, adding Durkan himself had struggled to explain his "absolute stupidity" that night.
He remains on bail and is due to return to court to be sentenced on November 19.
Australian Associated Press