Your friend took defensive action when two robbers robbed with a knife, and stopped to call the police after the two robbers lost their ability to act, which was justifiable defense.

Article 20 of the Criminal Law In order to protect the state, public interests, the person, property and other rights of oneself or others from ongoing unlawful infringement, and to stop unlawful infringement, thus causing damage to the unlawful infringer, it belongs to self-defense and bears no criminal responsibility.

If justifiable defense obviously exceeds the necessary limit and causes great damage, criminal responsibility shall be borne, but the punishment shall be mitigated or exempted.

Taking defensive actions against violent crimes such as assault, murder, robbery, rape, kidnapping, etc., which seriously endanger personal safety, and causing casualties to illegal infringers, is not excessive defense and does not bear criminal responsibility.