Don't give her the oxygen of publicity or feed into her thinking there would be an issue.  Just treat her how you would have if she's any member of your circle of friends.

Bugg88 wrote:

Thanks mb16, Keeping it short does seem like a good plan.

Ori97, There are a million things I could say to her, but I don't even want to go there. I am really focusing on staying positive. I know that their relationship is just a manifestation of my past thoughts and that my current state of being is leading me towards reconciling with my ex.  I would like to go to the party and have fun celebrating my friend. There will be many other friends there could potentially make it awkward by bringing it up or whispering about it, but that's on them.

My response to her text would be ' Hi, it sure is going to be a little awkward but nothing that we both cannot handle. We are adults , right!
You must go to the party and have fun and maintain a distance with her. That's kind of normal & expected. But how elegantly you handle it on that day is something which you have to prepare yourself with.
I think if you do manage to ignore and be completely involved in just having a good time at the party, your small battle is won. All the best!