Wow, Kelly, that is a big question.
I think that the best way to guide your D would be to share from your own experience, rather than "preach".
I would share with her some experience that YOU had that was similar.I would share your heart and your pain when you did nor act wisely ,at times in your life.
I think that that is the best chance that you have to get through to her.
'Preaching" will make her run the other way.(IMO). The other thing that I try to remember with my sons is that most things pass. It sounds like she is a good kid who is just having poor judgment right now. Am I right?My S(older) had a huge crush on his store manager who was old enough to be his mother--bleh.
We "walked" through it together. At times, I thought that he would succumb( and I know that she would have) ,but I just talked to him with as much wisdom as I could about what the consequences would be( he would be beaten up by the husband--lol)
My S and I went through this drama for a year.I didn't preach,but just tried to share my life experience . I told him that he had to listen to himself b/c I could not be there with him. I told him that what you reap you sow and maybe he did not want to sow what would come later.
After a year ,he found a g/f his same age. So, I did not panic,but just tried to use wisdom and he walked through unscathed.
Keep Sharing,Kelly..I am sorry for the pain of this situation for you.
Love Ami
(((((((((((Kelly))))))