Louella Fletcher-Michie, the daughter of Holby City and Coronation Street actor John Michie, died after taking 2C-P during the music festival, in September 2017.
Her boyfriend, Ceon Broughton, was convicted of manslaughter by gross negligence and jailed for eight-and-a-half years in March 2019.
Winchester Crown Court heard that he supplied Fletcher-Michie with 2C-P at the festival in Dorset, and then filmed her death rather than seeking medical help.
She was found dead in the early hours of 11 September 2017, the day she should have celebrated her 25th birthday, in a wooded area at the edge of the festival site.
Broughton, 31, of Enfield, challenged his manslaughter conviction and his jail term at a Court of Appeal hearing in June.
Lord Chief Justice Lord Burnett, Mr Justice Sweeney and Mr Justice Murray quashed his conviction on Tuesday.
They said that in the case, “expert evidence was all that the jury had to assist them in answering the question on causation” for Fletcher-Michie’s death.
“That expert evidence was not capable of establishing causation to the criminal standard,” the judgment added.
“In respectful disagreement with the judge, we conclude that the appellant’s main argument, that the case should have been withdrawn from the jury, is established.
“The appeal against conviction for manslaughter must be allowed.”
Seven years of Broughton’s sentence was for the manslaughter conviction, and the Court of Appeal has not ordered a retrial.
His barrister, Stephen Kamlish QC, argued the conviction was unsafe because prosecutors had failed to prove that Fletcher-Michie would have survived had she received treatment by a certain point.
He also argued Broughton’s sentence was “excessive” in any event.
Broughton previously admitted supplying drugs to Fletcher-Michie at Glastonbury festival earlier the same year, and was in breach of a suspended prison sentence imposed for possessing a lock knife and a Stanley knife blade.
The court heard he did not seek help for his girlfriend because he feared being arrested and sent back to prison, despite being just 400 metres from the festival’s medical tent at the time.
Prosecutors said Broughton filmed his girlfriend’s “gradual demise”, which lasted several hours, as she screamed and collapsed.
The jury was told that when a friend told him to call an ambulance, Broughton messaged back, “I can’t get bagged [arrested].”
Broughton did not give evidence at his trial. His defence barrister said he felt unable to leave his girlfriend alone in the woods while she was suffering what he thought was a “bad trip” and he had not realised she was at risk of death.
A statement read during his sentencing hearing said: “Sorry I didn’t do more to save Louella, sorry for the suffering I caused to everyone who loved Louella, I want to make things right.”
The Court of Appeal ruling said Fletcher-Michie had taken her own drugs to Bestival but asked Broughton to bring more after they were confiscated by security.
The couple took drugs together on the afternoon of 10 September and went into woodlands, where Broughton took initially consensual photos and videos of his girlfriend to record her “trip”.
He contacted her mother, friends and relatives when her condition deteriorated, but festival officials went to an area called the Ambient Forest rather than the woods.
Fletcher-Michie is believed to have died at around 11.30pm and her body was found by security guards, who were alerted by Broughton, at 1am.
The trial judge, Mr Justice Goose, told Broughton: “You were only concerned for yourself. You didn’t want to be arrested, you were more in fear of that than in getting help.”
But the Court of Appeal judges said he had “made attempts to get assistance”, including by telling a friend to “get the medics” and sending him a pin on Google Maps.
The ruling said: “It is not plausible to suppose that [Broughton] was acting in a grossly negligent way whilst actively seeking help for Louella at that time.”
Judges said that for the conviction to be safe, the jury would have had to be sure that “at the time when Louella’s condition was such that there was a serious and obvious risk of death, the appellant was grossly negligent in failing to obtain medical assistance and that such assistance would have saved her life”.
Traces of 2C-P, MDMA (ecstasy) and ketamine were found in her body, but the Court of Appeal said a “definitive cause of death could not be established save that it flowed from the ingestion of the combination of drugs”.
Additional reporting by PA