Sharnita...I agree that this will be the last big event as they are graduating and she can get away from this girl, but if the girl almost ruined her homecoming experience by attaching herself to DD, she will do the same thing at the prom, which is a once in a lifetime experience. I think the girl needs to be told, up front, that she is not part of the group. It can be done nicely but she has to be told in no uncertain terms that they are not prom dates!!! OP's DD should not have to run and hide, nor should her friends have to run interference because this girl is obsessed with this make believe friendship. Yelling "Get away from me!!" is not recommended, but if this is all that is going through OP's DD's head the whole prom, she will not have any fun.
I'm not sure it can be done nicely (or at least gently) and still have her get the point. And I think knowing that they hurt her feelings deeply culd also almost ruin the event. I think it is a crummy situation for DD but sometimes the reality is that either way you have choices you aren't going to like, through no fault of your own. This girl is not taking hints, even broad ones. They could broadside her with the painful truth but she would be hurt and they would be upset because they hurt her which would just result in a bad night all the same.
I agree that it can't be done *effectively* without hurting this girl's feelings, but it still can be done nicely.
And the OP's DD and her friends need to realize that they do NOT have to worry about not hurting her feelings at all.
They only need to worry about not GRATUITOUSLY hurting her feelings.
So the DD can say, "I'm sorry--I have plans to spend the night with a group of my friends. I can't include you."
She should say it right away. And pleasantly. Without *too* much regret.
Your DD needs to internalize the idea that she is not being rude--that this girl is in error. Sort of like, her zipper's undone or something. That this girl has wrong information and your DD is actually honor-bound to *correct* it. And that's what she's doing--correcting her, not rejecting her.
It will help if she also internalized the idea that the "party" is not hers alone, and so she is not authorized to include this girl in the party. (even if she is the one who would most object to her being included)
And your DD may need to gear herself up for saying, "I'm sorry, but I don't want to spend prom with you. I have plans already." (the second time she has to say this, she should add on, "with my friends" (implication being, you're not one of them)
(In fact, your DD *does* have "a date"--all the rest of her friends are "her dates," and so that's what she needs to say. "I have plans.")
Then, to make herself feel stronger (and better about herself), she should remember: Once she does say, "I have plans with my friends, adn I can't include you," this girl should get a clue. If she doesn't, then your DD is now in self-defense mode, and she is entitled to feel a little bit resentful.
(also, the sooner she does it, the better. It's kinder to the girl, for one. And easier on her because she'll have it over with and will have time to fix it so
Meanwhile, you and DD might role play some ways to completely discourage conversations with this girl at all--how to arrange so that your DD's lack of interest in a friendship is clearer.