The state’s highest court Wednesday upheld the involuntary manslaughter conviction of a 22-year-old Plainville woman facing 15 months in prison for coaxing teenage pal Conrad Roy III to kill himself nearly five years ago in a store parking lot.

Michelle Carter’s sentence was stayed pending the Supreme Judicial Court’s ruling.

“The evidence against the defendant proved that, by her wanton or reckless conduct, she caused the victim’s death by suicide. Her conviction of involuntary manslaughter as a youthful offender is not legally or constitutionally infirm. The judgment is therefore affirmed,” the court ordered in a 33-page decision written by Justice Scott L. Kafker, a former deputy chief legal counsel to Gov. William F. Weld.

Defense attorney Daniel Marx, who argued Carter’s case before the SJC Oct. 4, said he’s considering fighting Wednesday’s decision all the way to the U.S. Supreme Court.

Janice Roy, the victim’s grandmother, said she was relieved by the decision.

“It’s been a long time coming,” she said. “We’re happy with the outcome.”

Carter’s parents could not immediately be reached for comment.

“In response to numerous inquiries about what happens next in the process: I can inform you all that our next step in this matter is to file a motion to the juvenile court requesting that the stay of sentence be revoked and the jail sentence be imposed. We will be filing our motion with the court in the coming days,” Bristol District Attorney Thomas M. Quinn III’s spokesman Gregg Miliote said in a statement.

The SJC said Carter was aware Roy had been researching ways to take his life and that she “helped plan how, where and when he would do so, and downplayed his fears about how his suicide would affect his family. She also repeatedly chastised him for his indecision and delay, texting, for example, that he ‘better not be (expletive) me and saying you’re gonna do this and then purposely get caught’ and made him ‘promise’ to kill himself.”

The justices concluded Roy “was badgered back into the gas-infused truck by the defendant, his girlfriend and closest, if not only, confidant in this suicidal planning, the person who had been constantly pressuring him to complete their often discussed plan, fulfill his promise to her, and finally commit suicide. And then after she convinced him to get back into the carbon monoxide filled truck, she did absolutely nothing to help him: she did not call for help or tell him to get out of the truck as she listened to him choke and die.”

“We are disappointed in the court’s decision, which adopts a narrative that we do not believe the evidence supports,” Marx said. “We continue to believe that Michelle Carter did not cause Conrad Roy’s tragic death, and she should not be held criminally responsible for his choice to end his own life. Today’s decision stretches the law to assign blame for a tragedy that was not a crime. It has very troubling implications, for free speech, due process, and the exercise of prosecutorial discretion, that should concern us all.

“There are good reasons why nearly every other state has passed a law to address ‘assisted suicide,’ which inevitably involves complicated circumstances better addressed as a matter of policy by the Legislature than in any particular case by the court. We will evaluate all legal options for Michelle, including a possible appeal to the U.S. Supreme Court,” he said.

Carter was 17 when Roy, 18, poisoned himself to death in the parking lot of a Fairhaven Kmart on July 12, 2014, by filling his pickup truck with deadly carbon monoxide fumes while talking with Carter by phone and text messages. When a deeply depressed Roy had second thoughts about dying, prosecutors said Carter ordered him to “get back in” the pickup.

Roy, a Mattapoisett resident, had just graduated from Old Rochester Regional High School and had already been accepted to Fitchburg State University. The only son born to Conrad H. Roy Jr. and Lynn Roy had also earned his captain’s license from the Northeast Maritime Institute. In addition to his parents, he was survived by two sisters.

Carter also faces a wrongful-death lawsuit brought by Roy’s mother in Norfolk Superior Court. That case is next scheduled to be heard March 6 for a litigation-control conference.

Bristol Juvenile Court Judge Lawrence Moniz sentenced Carter to serve 15 months of a 2½-year sentence after finding that her reckless failure to take any action that could have saved Roy’s life was the catalyst for him going through with the plan.