JOEL - First of all I assure you that your Mormon friend is also Christian. She also believes in Christ. She also tries to live her life in a Godly manner. At least I assume she does. While some of her beliefs may differ from yours, her basic desires and devotions are to Christ and His Gospel. Of course, any marriage counselor will tell you that the success of a marriage is more certain when the couple shares common cultural and religious backgrounds. But that doesn't mean that the marriage won't work if you are both unselfishly devoted to each other. To those of us who are members of the LDS Church there is no difference between LDS and Christian. We believe Christ's church was reestablished by Christ Himself on this earth through the founding prophet, Joseph Smith. Joseph learned directly from the source what it means to be a follower of Christ when God and Jesus appeared to him back in 1820.
There are, however, points of doctrine in our religion that are different from the "Traditional" Christian beliefs. A big one is that we believe that a man and woman can be married, not just for "until death do you part", but even after we die and for eternity. This type of marriage, however, can only be performed in our temples for worthy couples who are both members of the Church and who have lived the teachings and doctrines of the Church of Jesus Christ. From our youth up we are always taught and encouraged in our church to plan to marry in the temple when we are older. It is one of the most critical steps in our journey to eternal life with God in heaven. But it is somewhat difficult for those outside our church to understand how important it is that we marry in the temple. And it is equally difficult for those within the church to accept anything else but a temple marriage. A lot of it depends on how devoted your friend is to the Church and Gospel of Christ and what she is willing to give up to marry you. She would have to give up the chance to be exalted in the highest level of heaven, and her family life would have to exist without the benefit of having the priesthood of God in her home. More often than not when a member marries a nonmember, they eventually become inactive in the church and their children never become baptized. Sometimes members marry a nonmember, hoping that someday their spouse will join the church as well. Sometimes that happens; sometimes not.
I have seen a few member/nonmember couples who are very happily married, where the nonmember is very tolerant and accepting of the other's beliefs and does not interfere in her religious worship. But it can still put a strain on the relationship, unless the couple are completely unselfish and devoted to each other's happiness. But it is not my place to tell you what you should or should not do. These kinds of decisions have to be made between you and your friend. Your love for her should be unconditional. But think about some of the reasons why you like her. Chances are she acquired those traits as a result of her gospel-centered life. If you plan to be a part of her life, you owe it to her and yourself to find out all you can about the church through reading about it and talking with the missionaries. You may never decide to join the church, but at least it will help you to understand how important it is to her. God does love you both, and does want what's best for both of you. What's best for both of you, and all of His children, is to be married for time and eternity in His Temple.