Whatever your leaders have said, consider their counsel, give it the weight it deserves, then counsel with your Heavenly Father about your own situation. This girl is also a real person, not a caricature of a cult member, and even if the ultimate result is a breakup, she deserves to be treated with compassion and respect. Not sure I want to marry one and live a lonely life, I just want a life partner that can be there for me and any future children. Pants-to-Church Sunday left me a bit bruised. As a Christian one's values won't allow an affair on the lonely spouse' part either - but that probably won't bug you by the sounds of it.
So it is going to be over anyway. How church discipline works. The fact that she's planning to go on a mission should help. One of the most well known church prophets stated that he would rather see his own child dead in their coffin than married outside the temple. That is a hard truth. Ask her if her parents buy their underwear from a bookstore. He has no vices, is the happiest person I know, is a healthy role model of manhood for my teen daughter and loves me to the depth of his soul. I worked my butt off and supported our family through internship, residency and two fellowships always looking forward to the day when he would finally be in practice and things would get 'easier". There are a million fish in the sea and it makes no sense to choose one with whom you are not religiously compatible. Interreligious marriages are not a new thing.
And that my friend, is just a hint of the extent of the brain wash that Mormons experience. We are only engaged and as much as I love him, I can't handle the pain of watching my goals and dreams wash away to be at his disposal. You should ask yourself if you want to pursue a future partner who was raised in an environment that causes drastic sexual suppression and you may never have a healthy sex life if she is your wife. Willl he build resentment at the struggle to get him to change whether real or imagined. Thirty years ago I converted a guy and then dumped him for a returned missionary from an established LDS family. Doctors want to be with their families and attend events but because of work they just can't.
However, from what I have seen he has all of the qualities I want from someone long-term. Either way, if you have kids and you don't convert, she will divorce you and take your kids away from you and you will be shunned from her community. Looking back, I can say that when I received this answer to my prayers, I was at one of the most spiritually high moments of my life. And for mormons, the goal is always a temple marriage and a marriage for the eternities.