A judge said Kevin Howe, 20, “took some delight” in knowing what his inseparable friend Stephen Walker, now 31, was about to do, moments before he turned himself into a human fireball.
Mr Walker suffered 95 per cent burns and has undergone numerous operations since the incident at his home in Bishop Auckland, County Durham, in May.
Judge Christopher Prince, sentencing Howe at Durham Crown Court, told him: “He would not have suffered these injuries had it not been for your actions.”
The case was the first of its kind since the Director of Public Prosecutions published guidance in 2010 in the wake of an appeal by MS patient Debbie Purdy, from Bradford. She asked for clarification on whether her husband would be prosecuted if he helped her travel to Switzerland to have an assisted suicide in that country.
The judge said the facts of this case were unique.
Mr Walker was known to be suicidal, especially when drunk, and the pair had been on a heavy drinking session that day.
Howe was sent out on his bike to buy £4 of petrol and an 89p lighter from a garage and he made a 1.4-mile round trip to get them.
Mr Walker poured petrol over his head and began flicking the lighter, causing fumes to ignite as neighbours tried to convince him to stop. By this time Howe had left, knowing what was about to happen.
Mr Walker set himself on fire just as neighbour Lee Banks left the house, having bravely tried to rescue him. The windows shattered and Mr Walker ran outside, screaming.
One witness reported flames 1ft above his head, while another said he was unrecognisable because the fire had engulfed him.
Locals tried to put out the fire with wet clothes, and a passing police officer helped before paramedics arrived.
The judge said Howe could have simply refused to get petrol for his friend, whom he knew better than most.
Only 11 days earlier, Mr Walker had told someone he intended to set himself on fire, and Howe agreed he was capable of doing it.
The judge told Howe: “Had you felt any compassion towards Stephen, you would not only have refused to go, you might have offered him some support and comfort for the dark mood that he had entered. But you did not.
“You took the positive action to assist him.”
The judge added: “Perhaps what is most cruel and shocking is that you appear to have taken some delight in what was to occur after you left. Thus, your comment as you left – ‘He’s inside with the petrol, I’m going home out of the way’.”