The grandfather of an 18-year-old Mattapoisett man who killed himself in a landmark texting-suicide case lashed out at Bristol Sheriff Thomas M. Hodgson for the early release his grandson’s girlfriend Thursday before her jail time is up, saying Hodgson should serve the rest of her sentence.

“The sheriff should serve the rest of her time,” Conrad Roy told the Herald, referring to Michelle Carter. “He lets her go because she’s a good girl? She’s not a good girl.”

A spokesman for the Bristol County Sheriff’s office said the timing of a prisoner’s release is directed by state law and is not subject to the discretion of an individual sheriff, jail or even the Department of Corrections.

Carter, a Plainville woman convicted of encouraging Conrad Roy III to kill himself, is being released from jail on Thursday — almost four months early — thanks to good behavior, officials said.

Carter was convicted of involuntary manslaughter and sentenced to 15 months at the Bristol County House of Correction for her role in Roy’s 2014 death. Prosecutors proved that through text messages and phone calls, she ordered Roy to “get back in” his carbon monoxide-filled truck in a Kmart parking lot in Fairhaven after he expressed second thoughts about taking his life.

Bristol County Sheriff’s spokesman Jonathan Darling called Carter a “model inmate,” saying she was able to shave four months off her sentence by attending programs, working in the jail and staying out of trouble.

“Inmates in Massachusetts can earn up to 10 days off of their sentence every month,” Darling said.

Carter began serving her sentence last February and in September was denied early release by the parole board. Board members in their decision said Carter displayed a “lack of empathy” by “facilitating” Roy’s death and determined her release was “not compatible with the best interest of society,” according to a record of the decision.

Carter tried to avoid jail time altogether following her conviction in 2017. Her lawyers were successful in staying Carter’s sentence as they appealed her conviction to the state’s Supreme Judicial Court. That stay was revoked when the state’s high court upheld her conviction.

Carter’s lawyers tried to keep her out of jail while they appealed the case to the U.S. Supreme Court on the grounds that her involuntary manslaughter conviction violated the First Amendment guarantee of free speech because it was based solely on words that she texted or spoke. A judge denied the motion to delay her incarceration last February and Carter began serving her sentence that same month.

The Supreme Court last week declined to hear Carter’s appeal, upholding her conviction and sentence.

Carter was 17 when she convinced Roy to kill himself. He had been accepted to Fitchburg State University before he fatally poisoned himself on July 12, 2014, by filling his pickup truck with carbon monoxide while communicating with Carter by phone and text message.

The first-of-its-kind case gained nationwide attention, even inspiring an HBO documentary. It has also triggered a bill aimed at cracking down on suicide coercion. “Conrad’s Law,” named after Roy, would make convincing or manipulating someone into taking their own life a crime punishable by jail time, with a maximum sentence of five years. Roy’s family is supporting the measure.