The incident happened in October of 2007 at a White Castle restaurant on Chicago's South Side.
Prosecutors say Darius Walton accosted a woman who refused to buy drugs from him. When 23-year-old Ronald Heard, Jr. tried to help her he was shot and killed by Walton.
Heard was the son of a Chicago police officer.
The man convicted of giving Walton the gun is serving a seven-year sentence.