What is the ruling on Muslim assistance when one of their dueling champions is aided by the enemy?

Primary text

If the enemy aids their duelist against a Muslim champion, the Muslims must aid their champion and fight those who assist the enemy, but they must not fight their own champion. If the Muslim champion solicited help or showed pleasure in the enemy's action, he voids his guarantee of safety, and it becomes permissible to kill him. This is supported by the historical actions where Hamzah and Ali aided 'Ubaydah ibn al-Harith against Shaybah ibn Rabi'ah after 'Ubaydah was heavily wounded.