He enjoyed a long-neck of Coopers Red beer and pizza with his girlfriend of three years, Janie Panton Roberts, at their Marrickville unit in Sydney's inner-west.

A colleague at Duffy's work, where he was employed as a kitchen hand, had invited him to a DJ party at the Petersham Inn later that evening.

That colleague had also arranged to sell the 19-year-old the drug MDMA for the occasion.

By the next morning, 21-year-old Ms Roberts was dead after taking the MDMA capsules her boyfriend had given her.

Three months after the trauma of finding Ms Roberts gasping for air in the bed next to him, Duffy was charged with supplying the drug that played a role in her death.

Duffy, who has pleaded guilty to drug supply, appeared in Newtown Local Court on Thursday supported by his family from the Central Coast.

Ms Robert's mother, Kerry, had travelled down from Kempsey for the sentence, prepared to read a statement in court detailing the impact her daughter's death had had on her life.

The case was adjourned to January. However the court heard a toxicology report had confirmed Ms Roberts died as a result of MDMA congestion.

A statement of agreed facts tendered on Thursday detailed how Ms Roberts began vomiting after returning home from her night at the Petersham Inn.

The pair arrived at the event just before 9pm. An hour and a half later Duffy's work colleague arrived and supplied Duffy with seven MDMA capsules throughout the night.

Duffy had four capsules for himself and gave three to his girlfriend "without financial gain", the facts state.

The event finished at about 2.30am and the couple left.

Ms Roberts vomited outside the venue and threw up again when the couple got home after 3am.

Duffy told police while Ms Roberts fell asleep, he couldn't so he rode his skateboard to the Sly Fox Hotel in Enmore.

He stayed outside talking to people before returning home about 5.30am to find Ms Roberts still asleep.

According to the facts, Duffy woke up at 8.45am on June 5 and saw his girlfriend "gasping for air". Duffy called out to a friend, who was asleep on the couch in the loungeroom.

The friend ran downstairs to a cafe to borrow a mobile phone and called 000.

As paramedics made their way to the Addison Road unit, Duffy carried out CPR on his girlfriend. However when an ambulance arrived, she couldn't be revived.

Duffy, who is on bail, will be sentenced on January 12.