“It is true that he died from his (burn) injuries,” Dr Owen Menagh, of the Moi Teaching and Referral Hospital in Eldoret Town, told the BBC.
Mr Cheptegei died last Thursday after being attacked four days ago and suffering burns to 80 per cent of his body.
Neighbors said they heard Cheptegei screaming before he ran for help.
Local media reported that Ndiema sneaked into Cheptegei's home in Trans Njoia County, western Kenya, carrying a five-litre can of petrol before carrying out the deadly attack.
According to reports, he was splashed with fuel when he targeted Cheptegei and was later engulfed in flames when he set fire to Cheptegei.
Police are treating Cheptegei's death as a murder and Ndiema is expected to be charged with the crime, with her ex-boyfriend identified as the main suspect.
Both men were admitted to Moi Hospital before they died.
Cheptegei's death shocked people around the world, with Ugandans saying she was an inspiration to them.
The 33-year-old Olympic athlete was the third athlete to be murdered in Kenya in the past three years, with police identifying an intimate partner as the main suspect.
In 2021, world record holder Agnes Tirop was stabbed to death, and six months later, Damaris Mutua was strangled.
Cheptegei was born on the Kenyan side of the Kenya-Uganda border, but after failing to make it big in Kenya, he decided to move to Uganda to pursue his athletics dreams and represent his country.
She joined the Uganda People's Defence Force in 2008 after taking up running and rose to the rank of Sergeant.
Her career includes competing at the Paris Olympics this year, where she finished 44th in the marathon, but her hometown people called her a “champion.”
Cheptegei is scheduled to be buried at her ancestral home in Bugwo, Uganda, on Saturday.
Attacks on women have become a major concern in Kenya, with a national survey showing that at least 34% of women reported experiencing physical violence in 2022.
