I agree with a lot of what other posters have said, it is her mother's house. I would never want to force anyone to do something they didn't want to do.
BUT, that being said, my thought was, that if I was her, 33, an adult and I had genuine friends that I wanted to see and my mother couldn't deal with it, I would find somewhere else to have the get together. If my mother wanted to be the petty one, then it should be her to miss the first gathering. I would do the "adult" thing with my friends and then see my mother after. That is, IF these were people I really wanted to see.
Yes, it is very telling about the relationship between mother and daughter, but it also tells a lot about how the friendship is going. Don't stress and don't count on her, just have fun however you feel comfortable.