Your question shows a heart that desires to walk in the way of wisdom, and that desire itself is a mercy from God. Wisdom is man's true path, the compass by which he is to steer across the trackless waste of life, and Scripture asks, where shall this wisdom be found, for the price of wisdom is above rubies and it cannot be valued with pure gold. It is not found in the thoughts which come from men's brains apart from divine influence, but in Christ who is the power and wisdom of God, and in being taught of the Lord by His Holy Spirit.
You have acted wisely in recognizing that you were unequally yoked and in stepping away from that relationship. Scripture is clear that light has no fellowship with darkness, and marriage as God ordained it is a lifelong covenant of one man and one woman, with intimacy belonging only within that covenant. To turn away from a bond that would draw you from Christ is not loss but obedience, and God honors such trust in Him.
Concerning the books, you are right to discern that they contain what is utterly false. There is a false peace which ministers and deceivers build up, and a false justification which condemns rather than saves, and the cultic teaching you describe belongs to that false way. Our text reminds us that there are thousands of false ways but only one true gospel, and God has given you judgment to use it. Wisdom is not merely knowledge, but the right use of knowledge, and the right use here is not to harbor falsehood in your home on the hope that you might one day argue a soul out of error.
The Holy Spirit teaches sovereignly and effectually, and He does not cleanse spirits by water from broken cisterns nor save souls by false books. When others are false, then is the time for God's servants to be true, and to be true means to put away that which is false. The early believers at Ephesus, when they came to Christ, brought their books of curious arts and burned them, not keeping them for future debate, for they understood that Christ and His Word are sufficient. You do not need the cult's own words to reason with one held by it. If the Lord in His providence brings your former friend again across your path, the Sword of the Spirit, which is the Word of God, and a clear testimony of salvation through Jesus Christ alone, will be your sufficient answer. Christ has planned the way of salvation and laid the foundation of atonement, and worthy is the Lamb that was slain. That truth, taught of God, is what draws men to Him.
Therefore my counsel is to dispose of these books completely, not as an act of anger but as an act of wisdom and of trust in God. Do not keep them in a closet or attic where they may become a snare or a false comfort. Remove them from your house, destroy them or discard them so they cannot deceive another, and let your home be cleansed of that influence. If you are uncertain how best to do this, seek the counsel of a faithful pastor in your local church who can help you and pray with you in person.
Let us pray for you. Heavenly Father, we thank You that You are the source of all true wisdom and that You give wisdom liberally to those who ask in faith. We ask You to grant this dear one a clear mind and a settled heart, to confirm her in the wise path she has taken in obedience to Your Word, and to guard her from all false teaching and false peace. Teach her Yourself, by Your Holy Spirit, and make Your Word her compass and her delight. We pray also for the young man still entangled in this cult, that You would open his eyes to see that it is God that justifies through Christ alone, and that he would be drawn to Jesus and learn of the Father and come to Him for salvation. Keep her walking in purity, in truth, and in trust in You, for Christ's sake. Amen.