Alan Henning, from Salford in northern England, was seized while on an aid mission to Syria last December.
"Alan is a peaceful, selfless man who left his family and his job as a taxi driver in the UK to drive in a convoy all the way to Syria with his Muslim colleagues and friends to help those most in need," his wife Barbara Henning said yesterday in a statement issued via the UK Foreign Office.
"When he was taken he was driving an ambulance full of food and water to be handed out to anyone in need. His purpose for being there was no more and no less. This was an act of sheer compassion. I cannot see how it could assist any state's cause to allow the world to see a man like Alan dying," she said.
Henning added, "I have been trying to communicate with the Islamic State and the people holding Alan. I have sent some really important messages but they have not been responded to".
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"I pray that the people holding Alan respond to my messages and contact me before it is too late. When they hear this message I implore the people of the Islamic State to see it in their hearts to release my husband Alan Henning," she said.
He is said to have stopped believing in Jesus and had instead embraced Allah.
The friend disclosed that Henning had no qualms about joining the aid convoy last December because he no longer celebrated Christmas.
Henning is also said to have given up alcohol and wanted to learn Arabic.
His wife's intervention came as British Prime Minister David Cameron prepared to urge other world leaders to act decisively against ISIS at a UN summit in New York this week.
Cameron will meet US President Barack Obama at the UN and could formally be asked to join America in airstrikes aimed at "degrading and destroying" ISIS.
In a YouTube video, Shakeel Begg and Haitham al-Haddad said there was no justification for holding the aid worker and that it was "totally Haram".