Talk to her... NOW!
DSis did this very recently. Emailed the father and told him that his son would be getting in touch because he wanted to and that, at 22, she no longer thought it appropriate that she had to wriggle and backtrack, verge on lying to her son in order to save his face.
She told DN that she and his dad had never really had a good relationship but that she supported him in wanting to get in contact and had emailed his dad just that one time and never would again. She never has!
What she really thought was that DNs dad would be a total let down and DN would need some support - which he did. But DN has never blamed his mum, he has told her that she was far more measured and fair than he now feels he could be!
So talk to your DD, tell her you have emailed her dad to let him know she wants to contact him and then make yourself the same promise... never contact him again. Let him and his now adult daughter meet and come of that what may.
You have no right to prevent her trying, but will always be there to pick up the pieces.
Good luck getting your head around it. I know DSis was a blubbering wreck, occasionally furious, and eventually has managed to let it go.
DN and his dad met once, his family, wife and daughters all make an effort, facebook and email, occasional phonecalls, invitations to family holidays. Dad does not!