This depends on the specific situation. If your friend goes to someone else's house and just insults them without resorting to violence, and he kills your friend, it is intentional homicide. If your friend goes to someone else's house and commits acts such as robbery, murder, assault, or kidnapping, the other person's behavior is self-defense. Even if your friend kills someone, it is not considered excessive self-defense. If two parties fight each other and one of them dies, it is not self-defense and the murderer is still guilty of intentional homicide.
If it is the first three situations, the murderer will not only bear criminal liability, but also civil liability, that is, compensation for your medical expenses
If it is the second situation, the murderer will not bear any liability any liability.