[The Ruling on Assisting One of Two Innovating Factions Who Are Fighting Each Other]

Answer – and Allah is the One who grants success: We can say that it is permissible to assist with wealth and weapons, without directly engaging in the fighting. Imām Amīr al-Mu’minīn ʿAlī, Peace be upon him, said to his son al-Ḥasan, Peace be upon him: “Do not fight the Khawārij after me, for the one who seeks the truth and errs in it is not like the one who seeks falsehood and attains it.”
As for assisting with wealth and weapons, it is no more than seeking the help of the openly sinful (fussāq) and the people of innovation in fighting against rebels (bughāt). Amīr al-Mu’minīn ʿAlī, (Peace be upon him), sought help in his battles from the Khawārij before they rebelled, and from al-Ashʿath ibn Qays, and so on.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2