On February 18, Hewitson was driving on Rentons Rd with two passengers.
At an intersection, the victim got out and sat on the boot.
Rather than attempting to persuade him to get back inside the car, Hewitson set off, turning right into Severn St and increasing his speed.
At 30km/h, the victim fell off and went skidding along the road.
He ended up in Dunedin Hospital for five days, receiving treatment for a fractured skull and numerous abrasions to his face and body.
The court heard the man was left with concussion, unable to work or drive.
"This was in some ways a slow-motion inevitability given the poor decision that you have made to allow your friend to remain sitting on the vehicle while you drove, and it's you who must accept responsibility for that decision," Mr Heale said.
"The consequences for your victim may have been much more significant than what actually transpired on the day. The risk you posed to him in particular, but also to other road users, was significant."
The court heard the friends remained on cordial terms and the victim had shunned police when it came to preparing an impact statement.
Police opposed the discharge without conviction on the basis that the potential consequences of a conviction did not outweigh the gravity of the offence.
Mr Heale, however, granted the application — "by a very fine margin".
Hewitson, who has worked at St Clair Golf Club for more than five years, was banned from driving for nine months and ordered to pay his victim $2000.
