Grubb had fallen off the back of the bike and Mitchell and his other two friends, whose names were suppressed, had taken Grubb back to his flat, leaving him in the care of somebody else, before going to a party.
That person became concerned about Grubb and phoned an ambulance.
Grubb's father, Jeffrey Grubb, read out a victim impact statement in court today.
He said his family did not blame Mitchell for the accident, as he was not speeding, and the three friends had all made their own decision to get on the quad bike.
But he was angry about the lack of after-care from Mitchell and his friends for his son.
Judge Turner also disqualified Mitchell from driving for two years, and ordered him to pay $2000 in emotional harm reparation.
The family said they would donate the money to charity.