There's also a difference between "someone who is abstaining from alcohol" and "someone who has made an explicit request for a dry party." FFS, she's the guest of honor! It's hard for her to say "I don't like bars, I'm staying home."

More generally, I believe polite behavior should be based on courtesy towards the people you are actually in the room with, not some abstract imaginary community-standard person. If you're having a party in honor of somebody with morning sickness, who asks to not have broccoli on the menu? It's rude to serve broccoli at that party. It doesn't matter how many OTHER parties where it might be polite to serve broccoli. Or how nutritious a vegetable it might be.