Let me get this straight...
You don't really know or care about this woman anymore. You're NOT upset that you didn't get an invitation to her wedding and you couldn't have gone even if you HAD. You have no hard feelings over a friendship that came to a natural end and are not interested in reconnecting with her in the future.
But you're upset that your parents are going to her wedding?
Sounds like you need to decide if you really care or not, R., because that's pretty inconsistent.
My parents go to a different church than I do. When I got married, they invited practically every single person from their church to the wedding (thankfully, it's a small church). People I had met maybe ONCE in my life we're at the wedding because they loved my folks and wanted to be there for them.
So it's not really surprising to me that your parents would get an invitation if they are church-friends with her parents. And it's not surprising that they're excited about it. Some people just like weddings.
So...what's the problem? If you're such a cool cucumber about everything, then why is it such a betrayal that your folks are going?
Okay, so she "hurt" you in the past. So what? According to you, you haven't had a real relationship with her since high school (which I'm guessing was over eight years ago)! Are you really holding onto hurt feelings for that long? Kids that age do stupid and hurtful things all the time, R.. I'm not trying to downplay your pain here, but you need to realize that you're both very different people than you were back then.
Look, all I'm saying is that it seems like you WANT to have a laid back attitude about this whole thing (good for you!), but clearly you don't.
So the way I see it, you only have two options:
1. Get over it.
2. Talk to your parents and make them feel guilty for not being "loyal" to you when all they want to do is support their friends and enjoy a nice evening out with a free meal and CAKE.
Not too difficult to figure out which option would be "handling it like an adult" is it?
So here's hoping you make the right call...and maybe buy a cake for yourself because that stuff is just freaking DELICIOUS!!!