I disagree. Grandma definitely WAS rude. She could very well have said "Oh, I'm sorry, I already committed to Grandkid's Birthday." Or she could have gone late (later?) to the anniversary party. Instead, she (well, they, since there were others involved) decided that not only were they not going to put in more than a token appearance at Birthday, but that the Birthday events (cake, gifts, etc) had to be rushed since they were taking most of the attendees with them. So in effect, they cut short Grandkid's birthday party because they got a better offer well after they committed to the first event.
Lack of a spine does not exempt or excuse rudeness.