Hi RunAwayDaughter,
You are living a parents worse nightmare and I am sorry you are having to go through this.
I read through your first posts and the one above where you state it is your daughter.
I am assuming she is 18?
If not then she is a minor but since you havent invoked the rights of parents of minors, I am guessing she is over 18.
I know it is not what you want to hear but usually in these situations, ANYTHING you do will result in her digging in more.
It sounds like you have limited communication with her. Let her know you will have a ticket (NOT MONEY) at the gate anytime she wants to fly home and then drop any and all efforts to "reason" with her.
When you talk to her share your love and mirror (validate) her feelings.
This will be the hardest thing youve ever done but its worked for other moms.
There is a saying "If you find yourself in a tug of war, lay down your end of the rope."
If you can think of ways to effectively do that it will let her see her situation in her own time.
Remember that half of all marriages end in divorce, in relationships that are not marriages, the demise rate is even higher. In 5 years she wont even be with this guy but based on how you handle things now, she will know she always has her mother to turn to.