The man waited outside a day care center in Hoboken, N.J. He had a question for the owner: How was he going to tell his daughter that she would never see her mother again?

His wife, Fabiola Bittar de Kroon, had been killed by falling debris after a commuter train barreled into Hoboken Terminal during the Thursday morning rush. She was the one fatality.

Now, hours later, her husband, Adrianus de Kroon, was at Smart Start Academy, a day care center not far from the train station. He met with its owner, Karlos Magner, outside.

“He said, ‘What should I tell her? How should I handle this?’” Mr. Magner said.

Mr. Magner had known children who had lost a parent to illness, he said, but never a death as sudden and violent as this. “I told him honestly: ‘I don’t know. There’s no book. Life, we’re not trained. We don’t know. So just stand strong,’” Mr. Magner recalled during an interview on Friday.

Ms. de Kroon, 34, had dropped her daughter, Julia, at the center on Thursday morning about an hour before the crash. Her life ended just as she was paving a new beginning in the United States with her husband and her daughter, whom they had 20 months ago in her native Brazil.

Mr. Magner said Ms. de Kroon left Julia at the center and picked her up every day, sending a flurry of kisses in her daughter’s way.

“Her daughter is attached to her,” he said. The girl grew restless on Thursday as the hour that her mother normally arrived to get her came and went, Mr. Magner added.

“She knew something was off,” Mr. Magner said.

Ms. de Kroon moved to Hoboken in April. She had planned to look for a bigger place for her family on the day she died. “Maybe a house in Brooklyn,” she had told a close friend, Roberta Lima.

Ms. de Kroon and Ms. Lima had communicated through cellphone messages over WhatsApp, a messaging app, on Tuesday, as they often did. Ms. Lima, 35, who lives in Rio de Janeiro, asked Ms. de Kroon when she would return to Brazil for a visit. Ms. de Kroon told her that she had planned a trip for December, in time to celebrate Julia’s second birthday.

Ms. de Kroon was “a bit apprehensive to be living far from her family now that she had a child,” Ms. Lima said, but living away from family was nothing new to her. She and her husband, whom everyone knows as Daan, spent four years in Miami before moving to Brazil in 2011, where he led global business development for a Jamaican beer and a Brazilian cachaça brand owned by an international distributor, Diageo.

Mr. de Kroon, who is Dutch, was working in the Brazilian city of São Paulo when he met his wife. Ms. de Kroon had grown up in Santos, on the Atlantic Coast, but moved to São Paulo for law school, Ms. Lima said. The couple moved to Miami in 2007, and they both studied business at Florida International University.

“They were very focused on their careers and very much in love,” Ms. Lima said.

Four years later they were back in Brazil. According to Ms. Lima, as the country’s economy sputtered, Mr. de Kroon moved to Asunción, Paraguay, to lead Diageo’s expansion there. Ms. de Kroon was pregnant at the time and decided to stay in São Paulo, close to her mother, Sueli Bittar. Mr. de Kroon would travel home on weekends, Ms. Lima said. Ms. de Kroon told her that moving to the United States would give the couple and their daughter a chance to spend more time together.

In April, Ms. Lima helped organize a farewell gathering for the couple in Santos. They went out dancing, had some drinks, had fun, she said.

“They were both very happy, full of plans, really excited about the move,” Ms. Lima said.

Now, Mr. de Kroon will have to be “both mom and dad,” Mr. Magner said. The only advice he could think of offering the widowed father was not to broach what had happened with his daughter until he felt the timing was right.

“Only the parent will know what is the right time and when to disclose it,” he said. “Children are like a sponge, they absorb so much, but they understand.”

Of Mr. de Kroon, Mr. Magner said, “He was strong, as strong as you can be in this situation.”

Ms. Lima and Ms. de Kroon were both 14 when they met, through a mutual friend. They last saw each other in Rio, at a birthday party for Ms. de Kroon’s twin nephews, shortly before she left for the United States.

When a bomb exploded in Manhattan recently, Ms. Lima sent Ms. de Kroon a message to make sure she, her husband and their daughter were all right.

“She told me, ‘Don’t worry, everything is fine,’” Ms. Lima recalled. “She said where they lived, on the other side of the river, was very safe.”