On the b-day party... use the standard question: Would it help, hurt, or have no effect?
And ONLY go if YOU can handle it... it might have been a knee jerk saying that he only asked you because OW is out of town... what if that were the case? What if he asked you to go and you knew OW would be there? And with OW not around, why NOT go?
Just some thoughts... but the answer truly is... do works and what's right... for you...
Seeing your last post though, as anticipated, you are going through your own roller coaster of emotions... It will take a while to get through them...
Best you can do is be as stable as possible while around your H. Set some boundaries to help with that, as necessary. And GAL... even when you don't quite feel like it... and also... understand it's also OK to watch a girlie movie and eat ice cream and bon bons when you need to... I did that...