” There are some “general principles” in Scripture that you can occasionally ignore and sometimes (note, sometimes) get away with (avoiding debt, getting lots of counsel), but the call to marry a sincere believer is one that you ignore at your peril.
It’s not just about morality; it’s about so much more.
Dating an unbeliever can hinder your relationship with Christ.
Is it a help or a hindrance to their spiritual health? Unfortunately, too many people do not take into account the extremely serious situation of children and their eternal destiny.
Yet, because of "love" and because they listen to their hearts over the Word of God, many people marry unbelievers anyway... If you are already married to an unbeliever then you need to love him/her as best you can, set a good example of being a Christian, and pray for your spouse's salvation.
That is something about which the church must be bold and clear so as to not leave any question about the matter.
As believers, our lives are to be above reproach, holy, pursuing purity and, as leaders, a model for other believers. The bigger question is why has your church allowed someone to lead who A) is not making good choices about basic relationship issues, and B) does not come to church often? Given “B,” we shouldn’t be surprised about “A.” What would we expect? Your friend already knows where you stand on her relationship choices. I don’t think it’s necessary to revisit that with her. As I see it, her choices are not the primary issue. I think what you need to do is get clear from your church leadership, maybe your youth pastor, what are the expectations of young leaders and how can they help the young ones grow spiritually.