Cardiff Crown Court was told how Colin Lewis smoked a cigarette then drove home as his friend's body lay broken in the dark 150ft (45 metres) beneath the Pontsarn Viaduct near Merthyr Tydfil, Mid-Glamorgan.
Lewis, 32, a video store manager of Brecon Road, Merthyr Tydfil, denied murder but admitted aiding and abetting Stuart Jones, 28, to commit suicide last January.
Mr Jones, a hospital porter who suffered bouts of depression, died from multiple injuries. His body was found the next day.
Detectives treated the death as suicide until Lewis walked into a police station four days later and told them: 'I killed him.'
Christopher Pitchford QC, for the prosecution, said Mr Jones, who lived in Cardiff, had asked Lewis 'on more than one occasion' to push him off a bridge.
The barrister said Lewis told police he had pushed Mr Jones but subsequently claimed he had tried to save him.
The Crown accepted Lewis's denial of murder but, jailing him for aiding in suicide, Mr Justice Scott Baker said: 'Had you been a true friend you would have tried to stop him rather than help him.'