Nicola Furlong's friend broke down several times as she gave evidence at the trial in Tokyo of the man accused of her murder.
The friend told the court she had come to get justice for Ms Furlong.
She could be heard sobbing as she gave evidence from an adjoining room via an audio link on the third day of the trial.
The witness, who has not been identified, studied with Ms Furlong in Japan.
She recounted how they both attended a concert in Tokyo on the night of 23 May.
The woman said they each drank a 500ml bottle of mixed vodka and orange before the concert, and shared another during the concert.
They were approached by the accused, 19-year-old Richard Hinds, from Memphis, Tennessee, and his friend James Blackston at a train station afterwards. They were also at the concert.
The witness said they were friendly and kind. They travelled together to Shibuya, where the two men said they would show the Irish women a good bar.
At the bar, she said she remembered having two drinks. She recounted how she blacked out after drinking two tequilas with Ms Furlong and the two American men.
The next thing she remembers is waking up in a hospital with only a "picture in her head" of her friend being placed in the ambulance.
She said she still believes a drug was put into her drink the night Ms Furlong died, despite knowing nothing had been found in her blood samples.
The witness told the court she had never been unconscious from alcohol before.
"I still find it hard to believe it was just drink. There may have been drugs that have not been detected. That's just my opinion," she said.
The woman said she was angry with the accused for taking Ms Furlong away from her and the Furlong family.
She had travelled to Japan, she said, to defend and get justice for Ms Furlong.
Ms Furlong, a Dublin City University student from Curracloe, Co Wexford had been studying and living 100km north of the capital, at Takasaki City University of Economics, as part of an exchange programme.