A 21-year-old New Hampshire woman has died and a 26-year-old New Hampshire man remained in critical condition in a Boston hospital Monday afternoon following a Fourth of July car accident on the Vineyard last week that left five people injured, three critically. A stunned family member who was at the scene but not involved in the accident described the tragedy to the Gazette, noting that they had come to the Island every summer.

Heather L. Laflamme of Berlin, N.H., died Saturday at Brigham and Women’s Hospital in Boston, a hospital spokesman confirmed on Monday. Officials at Beth Israel Hospital in Boston said Ms. Laflamme’s boyfriend, Seth Jones, 26, of Dummer, N.H., remained in critical condition. His mother Margaret Jones, 51, of Dummer, N.H., was listed in good condition at the same hospital.

Services are planned in New Hampshire this weekend for Ms. Laflamme, who was a rising senior and dean’s list student at the University of New Hampshire.

On the afternoon of July 4 Ms. Laflamme and three members of the Jones family were riding in a red 2012 Jeep Wrangler on Barnes Road that collided with a black 2009 Volkswagen Tiguan. The driver of the Tiguan was Benjamin T. Johnson, 19, of Cambridge. Mr. Johnson and Thomas C. Jones, 51, of New Hampshire and the driver of the Jeep, were both treated and later released at the Martha’s Vineyard Hospital. The other three were medflighted to the two Boston hospitals.

According to state police, who are still investigating the accident, the Jeep was traveling on Barnes Road just after 2 p.m. on Wednesday when the driver began to make a U-turn and was apparently broadsided by the Volkswagen. The Jeep rolled over and Ms. Laflamme was ejected from the vehicle, police said.

In a conversation with the Gazette yesterday Donald Antonelli of Warwick, R.I., Mrs. Jones’s brother in law who was at the scene of the accident traveling in a different vehicle, recalled the events of that afternoon and issued words of thanks to all the people on the Vineyard who stepped in to help. Speaking from Beth Israel Hospital in Boston where he was gathered with other family members, Mr. Antonelli said the extended Jones family comes to the Vineyard every summer. And he said Seth, whom has known since he was a kid and “is like a son or brother to me,” is “fighting for his life . . . doctors are telling everybody to cross their fingers.”

The Joneses have four sons, Mr. Antonelli said, and Seth is a Marine who served his active duty in Afghanistan and Iraq and has been working as an apprentice welder at the Portsmouth Naval Yard. “He loves working for the Navy,” Mr. Antonelli said.

He said he was in a car driving behind the Jones family last Wednesday; all were headed to look at the War Veterans Memorial in Vineyard Haven and then to the Edgartown Fourth of July parade. Mr. Antontelli said his wife’s father is from the Vineyard.

He said the two cars were pulling out of the Airport Mobil station, with Mr. Antonelli, his wife, and the Jones family’s youngest son, 19, in a car behind the Jeep.

The red 2012 Jeep Wrangler turned out of the gas station onto Barnes Road, Mr. Antonelli said, and the car he was in, driven by his wife, waited for a car to pass before following. A car “flew by,” he said, fast enough that they commented on the speed.

“Three seconds later, we just see the Jeep flying in the air,” he said. His wife screamed and slammed on the brakes, he said, and they jumped out of the car. Another person who pulled up behind them jumped into the car to park it, he said.

Mr. Antonelli praised all who responded at the scene and at the hospital, from emergency responders to passersby. “Everybody was so helpful and kind and really heartfelt,” he said. “Thank you to everybody.”

He singled out for praise the help of one woman in particular whose name he said he did not get. “If she didn’t stop, I’m pretty sure Seth would’ve died in my hands,” Mr. Antonelli said. He described the woman as calm, cool and collected, possibly a doctor or a nurse. When she arrived, Mr. Jones said he was holding his nephew. “She kneeled down and said ‘Wow, he’s in bad shape,’ ” Mr. Antonelli recalled. “ ‘You need to keep him conscious.’ ”

He said he hopes to find out who she is. “I’d really like to go have lunch with her.”

Mr. Antonelli said there has been an outpouring of support for Mr. Jones, with friends and Marines coming to Boston from New Hampshire and as far away as San Diego.

“It was a traumatic, terrible thing,” he said. “It changed our lives.”