Ms. Gui said that even as she grew increasingly suspicious of the men, Ms. Lindstedt tried to persuade her to work with them and explained “that was the best course of action, as the negotiations handled by the Foreign Ministry ‘didn’t seem to make much progress,’ ” Ms. Gui said.

Kurdo Baksi, a Swedish journalist who has worked with Ms. Gui to try to free her father, also went to the hotel, joining other Swedish and Chinese guests invited to hang around there by the businessmen. He said one of the businessmen had claimed to be from China, and the other from Sri Lanka. Mr. Baksi said he did not take part in the negotiations, but said the odd setting for talks worried him.

“After a while I felt that everything became very uncomfortable,” he said. “I’m afraid this could complicate things for Gui Minhai.”

Ms. Lindstedt, 58, became the ambassador to Beijing in September 2016. She began her diplomatic career in 1990, when she became second secretary at the Swedish Embassy in Indonesia. She went on to positions in Pakistan, then became ambassador to Vietnam, then Mexico. She was appointed climate ambassador in 2011, and was Sweden’s chief negotiator at the climate summit meeting in Paris in 2015.

It is unclear how Ms. Lindstedt became involved with the two men who met with Ms. Gui. Last week, the Chinese Embassy in Stockholm responded to an earlier, less detailed account of the meeting, published by a Swedish newspaper, by denying that it had any role in the talks. “The Chinese side has never authorized and will not authorize anyone to engage with Gui Minhai’s daughter,” the embassy said in an online statement.

A spokeswoman for the Chinese Ministry of Foreign Affairs, Hua Chunying, said on Thursday that she had no information about the matter.

Ms. Lindstedt is not the only ambassador in Beijing who has become entangled in controversy over China’s detention of foreigners. The Canadian ambassador was dismissed by his government last month after making contentious comments about Meng Wanzhou, the chief financial officer of the Chinese telecom giant Huawei, who was arrested in Vancouver in December at the request of the United States.

The Canadian envoy, John McCallum, surprised his government by saying that Ms. Meng had a good chance of avoiding extradition to the United States, a comment that critics said threatened to politicize the case.