Seine said Farrell, who has come to embody Boney M's hit single "Daddy Cool", had given a performance in Saint Petersburg on Wednesday, but "they told me that he was not feeling well."
"This morning, he did not wake up."
The Saint Petersburg investigative committee of prosecutors confirmed to AFP that the entertainer was found dead in his bed by an employee of the Ambassador Hotel on Thursday morning.
"There were no signs of a violent death," the committee said, adding: "The investigation continues".
Farrell, a Dutch national born in Aruba, was the sole male member of Boney M, which won global fame with catchy disco numbers like "Belfast", "Ma Baker", and "Rasputin" -- many of which won gold and platinum discs.
The group sold more than 50 million singles and 60 million albums worldwide, according to figures quoted by producer Frank Farian.
Farrell's true part in this success has been questioned, with Farian having stated it was in fact he who recorded most of Boney M's male voices in studio.
"Yes, in the beginning it was not Bobby Farrell (singing), but later it was," Seine told AFP.
Seine described the star born Alfonso Farrell as "quite a bizarre person. He had a big heart but he was explosive. He was a fantastic person."
Remembered partly for his impressive afro hairstyle and extravagant costumes, often with bare torso and tight-fitting bell bottom pants, Farrell continued to create disco music after Boney M split up in 1986, including remixes of some of the group's most famous tracks like "Rivers of Babylon", "Brown girl in the ring" and "Sunny".
Proclaiming pride in his Caribbean culture, he often fused disco with influences from Aruba.
"I want my music to make people feel good and to create sweet memories," the artist described by his agent as having "a big mouth and a big heart" is quoted as having said on his official website.
"The energy in my music has no limit; I want people to feel entertained and to hear the love that I have for creating music, translated into my songs."
Farrell continued to tour the world in 2010, giving performances in the Netherlands, Turkey, New York, Colombia, Finland and Slovakia.
"I like to look good on stage and to release all my energy in my shows. I still wear my glitter costumes and my girls (backup singers) always look glamorous alongside me," the artist says on his website.
He was to have travelled to Italy for a performance on New Year's Eve, and was preparing to release a new album.
Seine said his client had suffered from ill health for the last decade, "heart problems, shortage of breath and problems with his stomach".
Farrell lived in Amstelveen, near Amsterdam. He is survived by a son and a daughter.