You have heard the old saying that "love is blind"–and perhaps this is true in your daughter's case. Nevertheless, if you are honestly convinced that she's about to make a serious mistake, you need to do what you can to help her get past her blindness.
Let me begin by pointing out what you shouldn't do–and that is to get angry and argumentative with your daughter. It would be natural to react this way–but in all likelihood she would get angry also, and even more determined not to listen to you. The Bible wisely says, "A gentle answer turns away wrath, but a harsh word stirs up anger" (Proverbs 15:1).
Instead, tell her calmly and clearly that you love her–and because you love her, it hurts you to see her make what you think is a mistake. You might ask what she hopes her marriage will look like in five years, and urge her to ask herself if those hopes are realistic. I often get letters from people who thought their spouse would change once they were married–but it seldom happens.
Most of all, pray for your daughter and urge her to seek God's will for her marriage–and her life. God loves her and His will is always best. Christ can change her and guide her–and He will, if she will open her heart to Him.