Oh my goodness, you are in for a battle and I hope so much that I can help you with what I learned from our own experience. This is a long post but it's an overview of the strategies that helped us save our daughter when she went through this several years ago.
It took 2 years of our lives to get through a scenario very similar to your's, starting when our daughter was an 18-year old freshman at A&M. It was a very hard time for our family, it's even hard for me to dredge up the memories for this post, but we ultimately succeeded in preventing her from converting and today she is thriving and happy. You should know up front that the LDS church is a machine built for indoctrination, there is no group that is better at it than they are. The machine is well-oiled, secretive, and built to refute Christian theological arguments. As someone already said, on the surface, they do religion better than anyone else, they put forth a remarkably slick version of "perfect" families and communities. They are known for their tenacity with conversions and they often convert through romantic relationships. There is a reason they target 18 year old kids: These kids are legally adults and parents have no control over them, and secondly, they are able to isolate these young people because many are away at college. From our experience I can tell you that your daughter is the target and the objective is baptism. They will use multiple mechanisms to quickly and efficiently indoctrinate their target: Love-bombing from the boyfriend and his group of Mormon friends; missionaries will quite forcefully insinuate themselves into her daily life, and even elders of the church will reach out to serve a guiding "parental" role to reinforce the indoctrination material. The fact that your daughter is in this situation has nothing to do with the strength of the relationship you share (as someone suggested above) but as this process continues you should be aware that there will be a determined effort on their part to alienate your daughter from her family so that they can become her "new family". In Mormonism, only the members who hold temple cards and pay their tithe are the chosen ones who will go to Heaven, non-Mormons are seen as inferior and unworthy, so they will continuously try to prove to your daughter that she must accept the "true gospel" of the Mormon church and refute the "corruption" of mainstream Christianity. This principle inherently promotes the belief that non-Mormon friends and family are "flawed" because they have not accepted Mormonism, therefore she must surround herself with members of her new religion. It seems like you have a good grasp of the basics of Mormonism, but you should know that the Book of Mormon has none of their doctrine in it, you would need to refer to their "Doctrine and Covenants" and "Pearl of Great Price" to get information. I relied heavily on the non-profit group, "Utah Lighthouse Ministry", to get factual data, as well as the Mormonism Research Institute website. You will need many, many tools like this in your arsenal to win this battle because you are up against hormonal young love on top of everything else. The scripted and well-practiced indoctrination strategies being implemented by the missionaries will be putting loads of pressure on your daughter to convert, which is one of the saddest parts of the ordeal. Her boyfriend is most likely trying to get her to commit to a deadline for being baptized and will be adding daily stress to get her to agree. The stress and anxiety of all this took a huge physical and emotional toll on our daughter and it ruined her college experience for the two years that she was enmeshed in it. She had to be put on anti-anxiety meds near the end of the ordeal. We are very blessed that we ultimately got her back and have an even stronger bond with her as a result of the experience. She has said that our steadfast belief in her, even when our relationship got rocky, taught her the meaning of unconditional love.
If your daughter would be willing, I think our daughter would be happy to share her experience and I truly think this might be the most helpful thing you can do for her. Hearing a first-hand account from someone who went through it would likely open her eyes to what is happening. Our daughter is now 30 years old and has been happily married to a wonderful Christian man for the last 6 years. They are also Young Life leaders so I think she can help your daughter to see from the perspective of a young, Christian girl who has been there. Let me know if you would like to facilitate communication between them. I will pray for your family.
