My kids have all, in various ways, tried to maintain a relationship with their father. We moved together to my home country and he stayed in the UK where he is from. I moved in no small part because he is, to be blunt, a crazy person. The kids would go over for visits and he could somewhat keep it together during those trips. He has opted out of attending any important events such as high school or college graduations because, as he said after the first high school graduation, he "didn't get anything out of it." So, yeah, that's the kind of bullshit they've had to deal with and honestly it's been heartbreaking for them.
A few weeks ago he was visiting the children where they are here. One is in university, two have now graduated from college. It was what I now call his "Burning Bridges" tour. While visiting the oldest (DS) he became verbally abusive to him and when my DS asked him to leave (EX insisted that he stay in his 1 bedroom apartment) and he refused and it devolved to the point where the police were called. The children were devastated. JFC. But I also see some changes in their ability to accept who he really is, and to establish boundaries going forward.
Everyone needs the love of two parents but when one of them lacks the essential quality of empathy, as my EX does, it makes those relationships at best shallow. That's all they are going to get from him.
It's natural for your DD to try, and admirable for her to attempt to do so after such a long break in relations, but my guess is that she will learn at some point who he really is. When my kids tell me about some crazy thing my EX said, I refer to the parable of the frog and the scorpion and remind them, that he can't help it, this is his nature. It's their job to figure out how and if they want to have a relationship with that parent and to find a way to accept that this is all they will have in a father.
I admit, I have mixed feelings about this even though I am incredibly proud of my daughter. He has been out of our lives for 5-6 years and this is a step back into our family which I dont welcome. I want him to keep hurting, he doesnt deserve the grace that my daughter is giving him. I don't want to share future events with him. Selfish or just being protective? I am unsure.
And it's your job to step back and let it take it's course. I get being worried but she's 27 and she's trying to figure out something that is perfectly normal for her to do. She's offering him grace because there is a part of her that loves him. That's normal and fundamental to the human experience*. Be there to validate her feelings and experiences, but try to not add your own into that mix. She will be better for it and it won't further complicate her own journey.
*I have a lot of professional contact with the adoption industry and it is beyond common that when an adult adoptee discovers their biological parents they fall apart emotionally because there was a part of them that has always felt the longing for that person. It's incredibly profound, the parent/child connection, and it's universal.