The correct answer to this open question is the following.
In the context of the Roman Republic times, my advice would be to appeal to Roman law. Romans had clear legislation to resolve these disputes. However, those also were violent times when unfortunately for Romans, powerful military groups ambitioned more power and control of Rome.
My advice to my friend would be that unfortunately, violence is justified to save the Roman Republic, although this never has resolved any issue. Violence breeds more violence, and sooner or later, revenge is the name of the game. It is a vicious circle in which victim and perpetrator change roles, but there is never a winner. Violence never has been the option.