The investigation found Sharmeena Begum escaped from a detention camp in Syria and is illegally raising funds online for ISIS members.
A journalist from the Shamima Begum Story podcast posed as an ISIS sympathiser to contact the former school friend online after she escaped from Syria’s Al Hol prison.
Ms Begum was mocked by Sharmeena Begum for being a failure and a non-believer, and it was claimed she had ruined the image of the women who had joined ISIS.
Sharmeena Begum also said Ms Begum did not contribute at all and she was “just another individual, living off the benefits”.
She said the runaway schoolgirl only left her home when her husband was away as he would not let her out.
Ms Begum “just followed her friends into what became the biggest misery of her life”, the BBC heard.
Theories that Ms Begum had worked in Husba, the ISIS religious police, and made suicide belts were described as “such an insult”.
Sharmeena Begum went to school with Ms Begum in Bethnal Green, east London.
But in December 2014, she suddenly went missing and it was uncovered that she had joined ISIS.
Ms Begum, then aged 15, and two other friends followed two months later.
They had three children together, who all later died from malnourishment or disease — a one-year-old girl, a three-month-old boy and a newborn son.
Ms Begum left Raqqa with her husband in January 2017, but they eventually split up, as she claimed he was arrested for spying and tortured.
She was eventually found nine months pregnant in a refugee camp in Al Roj in February 2019 by a journalist from The Times.
Her British citizenship was revoked shortly after she was found.
Since then, Ms Begum has been in a battle with the British legal system. In February, she lost her latest legal challenge over the decision to deprive her of her British citizenship.