Sharmeena Begum, no relation, dismissed her former friend, who wants to return to the UK, for "living off benefits" and mocked her as a non-believer.
The BBC also found Sharmeena was fundraising online for members of the IS terror group, which is illegal.
A top Syrian commander fears money like this is helping IS to regroup.
Posing as an Islamic State (IS) sympathiser, a journalist from the Shamima Begum Story podcast contacted Sharmeena online after she escaped from Syria's Camp Hol prison for women who were with IS, and their children.
Sharmeena is still in Syria, in hiding and using a different identity.
She was a schoolmate of Shamima Begum, in Bethnal Green, east London, when, in December 2014, Sharmeena suddenly went missing. She had run away to join the self-styled Islamic State terror group in Syria.