Tracy Lee Dowling was the last person to see Cootamundra man Luke Doyle alive when he overdosed after injecting oxycodone with her and another man in Young in March 2012.

Dowling was convicted of manslaughter by criminal negligence in 2018 after a court deemed she failed in her duty of care by not calling for help.

A friend and a bystander had loaded an ailing Mr Doyle into the front seat of Dowling's ute with the plan to take him to hospital. By the time the friend retrieved a bag and jumper from inside the house, Dowling had driven away.

Instead, she drove across town to her house and went inside to sleep, leaving Mr Doyle in the front seat of her ute. The 20-year-old's body was discovered next to the vehicle by Dowling's mother when she returned from work the following morning.

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Judge Gordon Lerve sentenced Dowling to seven years' jail, with eligibility for parole in February 2024, in the District Court in May last year.

Her appeal, made on the single ground the conviction was unreasonable and unsupported by the evidence, has been dismissed in the NSW Court of Criminal Appeal. The critical issue in the manslaughter case was whether it could be proved beyond reasonable doubt that Mr Doyle was alive when Dowling drove away with him in the car.

Medical evidence heard in the trial and examined in the review of the case indicated that there was a "reasonable prospect of reviving" Mr Doyle if he had been taken to hospital.

Mr Doyle was virtually comatose when he was put into the vehicle by two other friends so he could be taken to hospital, and to Dowling's knowledge he had taken both Xanax and oxycodone.

Appeals court judge Justice John Basten upheld the decision on Friday. Dowling "well understood the condition Mr Doyle was in" as he was placed into her car, Justice Basten said, as well as why he was placed there.

A post-mortem later revealed Mr Doyle's cause of death to be acute multi-drug toxicity.

"It follows that [Dowling's] deliberate act of taking him to her home and leaving him without any attention let alone medical attention, was grossly negligent and significantly hastened his death," Justice Basten said in handing down his decision.

"It necessarily followed that she owed the deceased a duty of care because she voluntarily assumed his care whilst he was helpless, and by removing him from the possibility of others providing help, 'secluded' him."

Dowling's own evidence that she had "checked his vitals" when Mr Doyle was taken to the ute, as well as that of five others including her mother, was taken into account by the court.

The presence of vomit throughout the passenger side of the ute, with stains splashed across the door, floor, above the glove box, between the seats and on the roof, also supported the prosecution's case that Mr Doyle was alive when he came to be in the car.

Two witnesses didn't recall Mr Doyle having vomited in the car, and an early statement from Dowling indicated the same.

"Although Mr Doyle had vomited at the house in which the drugs were taken, no explanation was proffered as to how there came to be vomit so widely spread in the vehicle, if he was in fact deceased at the time he was placed in the vehicle," Justice Basten said.