Patrick Devillers, 52, was detained in Cambodia last month at China's request. Investigators believe that he was a close business associate of Mr Bo and his wife, Gu Kailai, and may be able to shed light on the couple's financial dealings.
Cambodia had previously promised not to extradite Mr Devillers without evidence that he had committed a crime.
"He said he is going there to be a witness," he added, saying that China had requested his release. "The French embassy supported this 100 per cent." China has allegedly guaranteed Mr Devillers will not be prosecuted himself while on Chinese soil in return for his cooperation.
Mr Devillers arrived in Shanghai on the same day that Laurent Fabius, the French Foreign minister, met with his Chinese counterpart Yang Jiechi in Beijing. It is not clear if the two men discussed his case, however.
The architect, who had lived quietly in a modest villa in Phnom Penh for several years, met the Bos in Dalian in the 1990s, when he was hired by Mr Bo to design part of the city's street grid.
After becoming close to the family, Mr Devillers set up a company with Ms Gu in the UK in 2000 named Adad, but it was dissolved shortly afterwards. Mr Devillers said the company was an architectural design firm that had never been operational.
