The man, who cannot be named, took part in the FreeMinds FreePeople project – which works with men aged 18-25 currently serving or recently released from HMP Nottingham.
During a session he came face-to-face with his friend’s killer – learning to forgive and see his rival as a fellow human.
He told programme volunteers: “I actually understood that I have a choice to make – for example if someone annoys me I have a choice.
"The last session taught me forgiveness – I was in the room with the same guy who killed my friend but I did not recognise him.
“While in the sessions I understood him as a person, I understood him for what he was as a person and I saw his remorse.
“I saw him as a human being - as one like me not labelled as a ‘rival’.
"On a break he came and asked me ‘you don't recognise me do you?’ We shook hands.
“I never thought I would be in the same room with him and not hurt him."
And when asked how it made him feel, he said: “A miracle. This was a miracle.”
Nottinghamshire's Violence Reduction Unit (VRU) has so far invested over £40,000 into the programme.
Its chair, Police and Crime Commissioner Paddy Tipping, said: "This project is having dramatic results and changing the way these young men view the world and other people.”