So what is self-defense? According to the criminal law, taking defensive actions against violent crimes such as assault, murder, robbery, rape and kidnapping that seriously endanger personal safety, resulting in casualties of illegal infringers, belongs to legitimate defense and does not bear criminal responsibility.
The key depends on whether the man who was killed seriously endangered your friend's personal safety. One more thing, self-defense must be aimed at the ongoing violence of the other side.
Whether it belongs to self-defense is decided by the court, not the public security. Of course, the police can do something in this regard.