Shaykh Fu’ad Al-Amri (hafithahullah) said, “…Helping your brother in this situation [if he expresses his wish to go and join ISIS] is by informing the [kuffar] authorities because you are preventing your brother from joining the Khawarij. This is one of the most dangerous things that a Muslim
could ever do because the scholars have differed concerning the Khawarij. Some of them said that the Khawarij are not even Muslims – they are kuffar.
So when you prevent your brother from joining the kuffar [the Khawarij] you are helping him.
Thus it is extremely dangerous for one to ever consider joining this deviant group. Whoever knows about a Muslim who wants to go and join them, it is his responsibility to firstly tell the people who are close to that person to give him advice and to not go [and join ISIS].
If that person does not respond to the advice then he goes up another level to somebody who is at a higher level than them, until he gets to the authorities – even if those authorities are non-Muslim [inorder] to prevent his brother from doing that great evil and Allah knows best.”
Posted by Abdul Kareem Ibn Ozzie