OP, X is going to do what he wants and you won’t be able to do anything about it, so debating whether it is right or wrong here is pointless. From your posts, it doesn’t seem like X was the most involved father for the first 7 years after the divorce, so expecting anything different now is kind of pointless. My following advice is colored from the experience of being the child of divorced parents who both went on to make new families with new spouses.
First, I think you should apologize to your DD for any negative opinions you have expressed about X. Explain that his behavior towards her upset you, but you shouldn’t have expressed anything other than you love her and are there to support her. Vent to your friends, therapist, online, but never to DD. When DD complains about X, say something like “I’m sorry you are hurting. How can I help you?”. Saying negative things about X to DD is going to hurt DD.
Next, encourage DD to discuss this situation with X and try to work it out with him. At this point, DD should stop texting with GF and have a private phone conversation with her father. I would recommend she make notes about what she wants to say, why she doesn’t want to go without BF and what she wants from X in relation to their relationship. If DD struggles to articulate this, it would probably be helpful to get her a family counselor to work this out or even mediate the conversation with X.
You haven’t elaborated on DD’s relationship with her paternal family, but if there is anyone she has a good relationship with, I would encourage her to find a way to keep regular contact with them. It isn’t your job to facilitate contact with X’s family, but if there isn’t anything negative or toxic in their behavior towards DD, it will benefit her to have those relationships, especially because it seems like X replaced her with his “new family with GF”.
My personal opinion is that X should pay for DD and a guest to attend his wedding. He created a situation where DD feels like she needs someone she’s comfortable with when visiting his family. He created the distance in his relationship with his child and failed to facilitate her becoming closer to his partner and future children. The first step in fixing this would be to show willingness to accommodate DD, and of course telling off GF. I have a feeling you are of the same opinion, but under no circumstances should you share that with DD.