Vascular surgeon David Nott helped the 16-year-old while working 24-hour shifts with medical charity Medecins Sans Frontieres (MSF) in Rutshuru.
The boy's left arm had been ripped off and was badly infected and gangrenous.
Mr Nott, 52, from London, had never performed the operation but followed instructions from a colleague who had.
The surgeon, who is based at Charing Cross Hospital in west London, said: "He was dying. He had about two or three days to live when I saw him."
It is not clear how the boy was injured. It was suggested that he had been bitten by a hippopotamus while fishing, but Mr Nott also heard that he had been caught in crossfire between government and rebel forces.
There were just 6in (15cm) of the boy's arm remaining, much of the surrounding muscle had died and there was little skin to fold over the wound.
Mr Nott knew he needed to perform a forequarter amputation, requiring removal of the collar bone and shoulder blade.
He contacted Professor Meirion Thomas, from London's Royal Marsden Hospital, who had performed the operation before.
"I texted him and he texted back step by step instructions on how to do it," he said.
Even then I had to think long and hard about whether it was right to leave a young boy with only one arm in the middle of this fighting.
"But in the end he would have died without it so I took a deep breath and followed the instructions to the letter.
"I knew exactly what my colleague meant because we have operated together many times."
The operation is only performed about 10 times a year in the UK, usually on cancer patients, and requires the back-up of an intensive-care unit. Patients usually lose a lot of blood during the procedure, BBC reported.
Mr Nott, from Fulham, west London, had just one pint of blood and an elementary operating theatre, but the operation, performed in October, was a success and the teenager made a full recovery.
The surgeon, who volunteers with MSF for a month every year, said: "I don't think there's more than two or three surgeons in the UK who can do this. It was just luck that I was there and could do it.
"I don't think that someone that wasn't a vascular surgeon would have been able to deal with the large blood vessels involved. That is why I volunteer myself so often, I love being able to save someone's life."
In the absence of intensive-care facilities, Mr Nott said he had personally monitored the boy's recovery from his bedside, tending his wounds.
"It was touch and go whether he would make it so when I saw his face on the MSF website afterwards, it was a real delight," he added.