The 15-year old was among 110 girls abducted in February from their school in Dapchi, Yobe state.
While all of the surviving girls were freed, Sharibu wasn't as she refused to renounce her Christian faith and convert to Islam.
On Monday, the Boko Haram faction known as the Islamic State West Africa Province (ISWAP) executed aid worker Hauwa Leman - the second International Committee of the Red Cross worker it has killed in less than a month.
ISWAP told local news site The Cable they would make Sharibu a "slave for life".
Meanwhile, the federal government has condemned the execution of Leman and Lai Mohammed.
The minister of information said the government did "everything any responsible government should do" to save both of the murdered aid workers.
The insurgents had threatened on Sunday to kill Leman by Monday.
Last month, ISWAP warned it would kill Sharibu if its ransom demands were not met.
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