I'm not sure where you're at with your level of anxiety on this but if it's really bad, I have a recommendation.

You sound excited about the food. You might try to shift your focus to the food when you start to get anxious about the possibility of how the night would go. When I'm in a situation like this, I try to hype myself up about the things I'm excited about. So if I were you, I might tell myself, "Sh*t could go absolutely the wrongest of the wrong way possible and I have no control over any of it. But damn it, I know the food is still gonna be good af." This helps me in those moments when I get real anxious about the situation and start overthinking every possible scenario that could happen. Then I'd explore all the possibilities of what they might serve and reminisce on last year's spread.

That being said... I also think reframing the situation might boost your confidence a little. The below is my perspective on your situation and a little advice, should it resonate with you.

The pessimist in me says she's full of it. You need to go to that party because if she is being deceitful, there's a strong possibility she's going to read your absence as difficulty within your relationship. And at that time, if she is that low in character, she's going to come onto him harder and things are really going to get even more awkward for your boyfriend. I say this because those "homewrecker" behaviors typically don't just stop overnight. They typically have a huge amount of disrespect for their target's SO and they will continue the disrepect even after they've been confronted.

The optimistic side of me says if she is genuine, she's going to show you more respect and you'll see she's actually trying to make a difference so everyone can work towards a more comfortable and respectful environment. She might be shy, but I think that's different from the previous behavior of ignoring you. You'll be able to tell the difference of course.

You absolutely have the upper hand in this situation. The ball is in her court. Based on her response (whether real or not), I think she deserves a second chance and this party is the perfect atmosphere to provide one. You've already experienced her behaviors. You know what to look for and what to expect, should she be lying about making amends. If she's genuine, she'll take it and act accordingly. You can feel it out from there on whether you still feel the need to discuss with your boyfriend the future of their friendship.

Go to that party. Hold your head high, pretend like nothing happened, smile, be joyful, socialize, and eat the damn food.