Kuram Shaikah Zaman, from Manchester, was killed in the tourist resort of Tangalle.
A friend, who asked not to be named, said Mr Zaman was on leave from his job in the Gaza Strip, where he worked for the International Committee of the Red Cross as a physical rehabilitation programme manager.
“He provided prosthetics for people who have lost limbs,” his friend said.
Mr Zaman and a Russian woman were attacked following an argument with a man at a hotel, according to reports.
Four men have been arrested in connection with the incident, including Sampath Chandra Pushpa Vidanapathirana, 24, the chairman of Tangalle village council, Sri Lankan police said.
A post-mortem examination showed Mr Zaman had been attacked with a sharp weapon and shot, according to reports.
The Russian woman, named as 23-year-old Victoria Alexandrovna, was being treated for serious injuries in a hospital intensive care unit, according to reports.
Tangalle is a popular tourist destination about 100 miles south of the capital Colombo.
The Foreign Office said: “Next of kin have been informed and we are providing consular assistance.”