It’s always a tough situation to witness to a non-Mormon who is dating a Mormon, and you are right to be concerned.

You will need to be very careful when speaking to a non-Mormon who is dating a Mormon because Mormonism is a deceptive counterfeit and your friend’s romantic interest will cloud his or her discernment.

Any attack on Mormonism may be interpreted as a personal attack against them. There is no such thing as sitting down and discussing "theology" or "history" with a Mormon in purely objective terms. If you criticize any part of the LDS Church, its history or teaching, you are attacking them—this is how they as Mormons are conditioned to think about it.

However, it will be good to expose him to some objective material about the LDS church, though this needs to be done with care so he or she feels like they are getting good, solid, documented material and not just "anti-Mormon" literature that seeks to attack and destroy. Some good sources for information on the LDS Church are:

http://mit.irr.org/category/joseph-smith

Unveiling Grace, by Dr. Lynn Wilder (former BYU professor).  A warm and engaging story of how this faithful Mormon family found about biblical grace and discovered truth which led them to abandon the deceptive religion of Mormonism that they had faithfully practiced for thirty years. 

Order the book here (discounted): https://irr.org/store/unveiling-grace

Listen to a short version (5 min) of Dr. Lynn Wilder's testimony here:  https://www.youtube.com/watch?v=SIj-bN_rMK0

Watch the video story here: https://www.youtube.com/watch?v=Dl0c5nl6u48

Listen to a first-person story on romance and Mormonism here: https://unveilinggracepodcast.com/ugp-029-anna-bannister-romance-and-mormon-conversion-process

A good, short personal testimony of Micah Wilder (Adam's Road Band) who found Jesus while on his Mormon mission. https://www.youtube.com/watch?v=D569x5TqtVk

As you bring up the subject of Mormonism, I would suggest you offer to study it together with them rather than telling them your mind is made up and now you expect them to come to the same conclusion.  That way, whatever you use to study you can raise questions and observations so they think it through themselves.

For more detailed information on dealing with someone being drawn into relationship with a Mormon:  http://mit.irr.org/someone-you-care-about-being-wooed-mormons

Bottom line, hormones are stronger than reason at this point. So prayer, patience and openness are your best weapons. Try to avoid putting him in a position where if he does come to his senses he will feel like you are going to say "I told you so." Mormons present well, they are sincere, dedicated people, and many live quite consistently with their beliefs. However, they do use deception in their proselytizing, leaving out many factual things about their past history. If you can help your friend see the organization's deception and help separate how that friend views the organization from how he or she views their girlfriend or boyfriend and their family, it will help them make objective judgments about the validity of the Mormon religion.

Feel free to contact us at IRR.org if you have any follow-up questions or comments. The other thing that may be helpful to your friend is to read some of the numerous stories of former members that we have on this website.