[The Ruling on Seeking the Help of an Oppressor to Recover a Right]

Answer – and Allah is the One who grants success and assistance: If the right is certainly established against the one who denies it or usurps it, or if, according to the Sacred Law, retaliation (qiṣāṣ) is established as deserved against a particular killer, then there is no objection to seeking help from the oppressor or from others. Allah, Exalted is He, has said: “Cooperate with one another in goodness and righteousness, and do not cooperate in sin and transgression.” [Al-Mā’idah:2] And assisting in such matters as those mentioned is among goodness and piety.
And He, Glorified is He, said: “And whoever is killed unjustly, We have given his heir authority, but let him not exceed limits in [the matter of] taking life. Indeed, he has been supported.” [Al-Isrā’:33] However, it is not appropriate to seek the help of the oppressor except when it is impossible to recover the right otherwise. But if it is possible to recover it without the oppressor, then seeking his help is not appropriate, because the oppressor may be excessive and go beyond the right into falsehood.
Yes, if it is deemed likely that seeking the help of the oppressor will lead to going beyond the right with respect to that person, then it is better to leave off seeking his help.

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