Just so I am understanding correctly, she was planning on coming to your wedding, but now that she is engaged, she is no longer going? I can see how that is disappointing, and then to top it off she booked her wedding for a weekend when she knew you weren't available. I think I can relate to both you and your friend. It was really frustrating for me to find a date that worked for all the people close to me... I had to switch dates for things like my dad had a conference scheduled or it was super bowl weekend, lol. Eventually, I was getting to the point where I was just like, screw it, I'm choosing this date and everyone else can just deal with it. I fell in love with a specific venue (even though I still have never seen it in person!) and was not willing to compromise on it. So, I can see where your friend is coming from... but like everyone else has already said, she cannot expect you to attend.
I can definitely also see why you're frustrated with your friend... my cousin was planning on coming to my wedding, but she has since gotten engaged and no longer has any interest in going. Not really sure why that is... maybe she'll be too busy with her own wedding planning or can no longer afford to go? Either way, it was a little disappointing. I'm trying to imagine if one of my best friends scheduled her wedding on a day I couldn't go, and sure, I would be upset. But I don't think I would necessarily be upset with her. I agree that you should go to the dinner and try to be happy for your friend. People can be a little weird when it comes to weddings, so you don't want to ruin a friendship over something like this.