If your goal is still to win her back, and you think you can keep your mouth zipped up tight, then being there for Xmas may be an opportunity to show off any changes you have made, and remind her what she is missing.
BUT - if you're no longer interested in reconciliation, or you think she still needs to deal with reality that she can't have her cake and eat it too, or if you think it will be too confusing for your child - then don't go.
If you don't want to go, then an appropriate answer might be: "As long as you are with another man, I cannot pretend and share holidays with you.. My answer would be different if you had given up OM and were sincerely interested in reconciliation."
I think the first year that my ex left, he was shocked that I didn't invite him to share Thanksgiving with me and the kids (this was 10 months after he moved out). The truth is, my mom would have scratched his eyes out!
Now that it's been almost 8 years and he's remarried and I've moved on, I WOULD be happy to invite him and his wife - let bygones be bygones - except for the lousy way he has treated me financially, and our kids, since the divorce.
I've found that it just works best to split and share holidays: One year I have them for Xmas eve, then they sleep at his house and have Xmas day there. The next we reverse. Thanksgiving we trade years (although early on they would have dinner at my place and dessert at his.)
It can be lonely at first but I always managed to work something out. One year at Xmas my mom (who lives with me) was also out of town, so after the kids left on Xmas eve, I spent the night on a second date with a loner guy I was dating (we didn't date long but are still friends, it was a lovely night) then went to a grown up party on Xmas day with another friend.
Use your best judgment. Every sitch is different in this regard. It can be hard to tell if she is just checking to make sure you're still available as Plan B, or if she sincerely is starting to think about reconciliation.
