If the two parties want to negotiate a breakup, definitely don't bring the third person along, for this will only make the other person angrier. The best thing is to have a friend that both parties trust to be there to keep things cool.
如果真要談判，不要帶著情敵出現，可能會激發對方更深的怒意，最好是兩人共同認識的朋友在場緩頰。