Thomas van Straubenzee was reportedly on the phone with Harry when he was robbed in a south London street.
Harry raced across the city to help. When he could not find van Straubenzee, the 27-year-old prince gave a witness statement at a nearby police station.
A suspect was later arrested and bailed.
Harry is an army helicopter pilot. He recently returned from a two-month training exercise in the United States.
The Associated Press contributed to this report.