We are truly saddened to hear about your ex-boyfriend's entanglement in a cult and the destruction it has caused in his life and your relationship. We will pray for his deliverance and for the Lord to open his eyes to the truth. We must also address the relationships you have and the issues uncovered.
Firstly, it is important to acknowledge and mention the power of prayer and that the only prayer that God will hear us by. We have been saved by grace through believing in Jesus Christ:
For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, that no one would boast. Ephesians 2:8-9 WEB
What we must address initially is more about the relationship described as an ex boyfriend. Any relationship shared between a man and a woman should focus on godly traits and encourage each toward Christ. Since a relationship has already developed in the past, the goal of pursuing any further relationship should be pointed towards reconciliation and the pursuit of godly marriage. This may not be feasible as there are spiritual forces at work impairing his judgment and he is actively distancing himself from friends and family. Bringing him out of the cult must remain the focus and primary concern. It can also be considered a positive that you had previously established a relationship as it is what caused you to reach out for help since you already care for him dearly.
Jesus Christ Himself wants all to be free from any type of slavery or captivity and has died specifically to free us from sin and any way we may be deceived or entrapped by darkness:
The Spirit of the Lord is on me, because he has anointed me to preach good news to the poor. He has sent me to heal the brokenhearted, to proclaim release to the captives, recovering of sight to the blind, to deliver those who are crushed Isaiah 61:1
So if the Son sets you free, you will be free indeed. John 8:36.
Let us pray for his God's intervention:
Dear Heavenly Father,
We come before You in the mighty name of Jesus, lifting up the ex-boyfriend of our dear sister. Lord, he has been deceived and is now ensnared in a cult, separated from his family, friends, and loved ones. We stand on Your word.
Put on the whole armor of God, that you may be able to stand against the wiles of the devil. For our wrestling is not against flesh and blood, but against the principalities, against the powers, against the world's rulers of the darkness of this age, and against the spiritual forces of wickedness in the heavenly places. Ephesians 6:11-12 WEB
Then God's people will know the Lords hand is powerful and God's people there are holy. God will live among God's people. Zechariah 2:5 WEB
Lord, we ask that You open his eyes to see the truth and expose the lies of the enemy. Give him the courage to break free from the bonds of this cult and return to You and those who truly love him. Protect him from any spiritual backlash as he makes his escape. We pray in the mighty name of Jesus.
Furthermore if the cult's leader is using witchcraft and exercising ungodly power, you can take heart that such an act is an antithesis to the gospel and counted as sin:
Therefore do not give up your confidence, which has a great reward. For you need to have patient endurance so that, after you have done God's will, you may receive the promise. Now in just a very little while, He who is coming will come and will not wait. But my righteous one will live by faith. If he shrinks back, my soul has no pleasure in him. But we are not of those who shrink back to destruction, but of those who have faith to the saving of the soul. Heb 10:35-39
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither the sexually immoral, nor idolaters, nor adulterers, nor male prostitutes, nor homosexuals, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God. 1 Co 6:9-10