Michelle Carter was convicted of involuntary manslaughter by Judge Lawrence Moniz on June 16 in a case considered to break new ground in a state that has no law against encouraging someone to commit suicide.
Her boyfriend, Conrad Roy, 18, was found dead from carbon monoxide poisoning in his pickup truck in a parking lot in July 2014.
Carter, who was 17 years old at the time of his death, is due to return to a juvenile court in Taunton, south of Boston on Thursday for a sentencing hearing scheduled at 2:00pm (1800 GMT).
Carter and Roy exchanged hundreds of text messages in which the young woman repeatedly urged him to follow through on his plan to kill himself, conceal it from his parents and lie to his mother.
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Some legal experts had questioned whether Carter's actions were enough to secure a conviction under involuntary manslaughter.
But Moniz said it was her instruction to Roy to get back into the vehicle -- during a telephone call after he stepped out -- and her failure to sound the alarm that was crucial to the conviction.
The judge agreed that Roy "took significant action" toward ending his life, but said he broke that chain by getting out of his vehicle and pausing to tell someone what he was doing.
He did the same thing during two previous attempts on his life -- telling first a parent and then a friend who got him help. But it was different when he told Carter, in July 2014.
"She called no one," Moniz said in June, despite knowing where Roy was and having his family's telephone numbers. "And finally, she did not issue a simple additional instruction: 'get out of the truck.'"
The judge rejected a theory of involuntary intoxication raised by psychiatrist Peter Breggin, a defense witness who testified that Carter's own medication would have hindered her state of mind.
"In reality, there are no winners," prosecuting attorney Katie Rayburn told reporters in June. "Two families have been torn apart and will be affected by this for years to come.