If it was a black belt in karate, the fight would not have lasted 15 minutes.
Secondly a real legit Dojo of Karate, normally a Japanese decent, would not graduate a student younger then at least of upper high school age to black belt. This being said that the student may have the skills of a black belt but not the maturity or mentally to hold the title. If this was a Korean decent of Karate, there may be a chance that he could have obtained black belt since the Koreans changed the ranking system a bit, along with the age requirements for black belt.
Thirdly if this were true, then the Dojo of which he obtained the black belt only did it solely on the fact for publicity and commercial reasons for the Dojo or to promote it in the industry.
Oddly enough you may hear stories or 4/5 year old's obtaining black belts but truthfully who does that?
Besides all those facts, if your friend truly did indeed obtain a black belt, he wouldn't need you to teach him how to fight alone, he would be utmost capable of fighting alone. So concluding that he probably isn't a black belt and your whole story can be totally false now.