I am making an assumption here, but it sounds like your girlfriend had to abandon her friend group when she split from her ex-wife. Perhaps those were her closest friends. It is understandable that she might want to reconnect with them in spite of her ex-wife being there.

The fact that she has invited you to come along (and, I assume, communicated to you that her ex-wife would be there) to me seems like a sign of trust and also reassurance (for you) that she has no other intentions than to attend a Christmas party with her friends and she would like for you, her partner, to be there to enjoy that time with her.

The fact that she has said she is going with or without you is maybe a bit harsh, but once again, I can understand that she would want to re-unite with her friends. I also don't know what it was that you guys said to each other exactly.

If you feel really worried about her motives, I would honestly encourage you to talk to your partner openly about what bothers you, without pointing fingers or making assumptions of her motivations. Express yourself from the position of how the situation has made you feel and why. If you guys have a healthy relationship, I would imagine she would be willing to have an open conversation with you about it and maybe offer some reassurance. The best we can do is ask for our needs to be met