Well... she seems to have used the element of surprise, and her knowledge about you and the kids, to her advantage. How she showed up, how she presented herself, the accusations in front of your kid, the carrot of her new life and gifts that she dangled in front of DD.... From an objective standpoint, it almost seems thought out and orchestrated---- especially given how long she's been entirely MIA.
Of all the people in the world, she probably knows how to push your buttons better than anyone. Given that she's shown little to no interest in the kids for so long, it's altogether possible that the show today was more to get to you, than it was about connecting with DD.
That's a sticky situation with the relationship between her and DD- and your involvement as a concerned parent. You've said that DD has a counselor? Perhaps the counselor might be the person to take this to? Then DD can get some help with her feelings about the situation that could be seen as less tainted by you?
Ugh, IDK, but am so sorry you're dealing with this.