I think as much as it galls you should be there for your dd, no one knows how the future will pan out, they may well be still married in 40 years time. Support your DD through her crazy mistakes, its nothing that can't be undone, and its not for you to give ultimatums.
What you need to do is make it clear you don't approve (which you have) but also that you will support her. Yes its crazy, yes it may well end in tears, YANBU to be absolutely against it, but I think there comes a point where if you're ever going to have a happy future with your DD and future grandchildren, you need to show that you can be there even when you're dead set against it.
I didn't marry, but I did move in with a horrible man when I was 19... my parents dispised him they made their feelings clear BUT they were also incredibly supportive. They waited 5 years for me to see sense, they picked me up when I was down even though it was of my own making, they gave continuous love and support and were extremely happy when I finally left the abusive bastard.
Knowing I had their love was very important, it didn't stop me making mistakes but it sure as hell helped me learn from them. My respect for my parents is immense, I put them through a lot. Had they thrown a hissy fit or given ultimatums, I'd probably still be with the arsehole, out of stubborness if nothing else.
I feel for you, I really do, but retain the moral highground, show by your actions that you accept your dd has the right to make her own mistakes and tell her you love her everyday. Even if she's a bloody nightmare! I was AWFUL as a teen, I'm 'nice' now, and that's down to my parents love and support.