At a venue, I think it's a bit different. So long as she didn't get in the way, I don't think it's outright rude. Someone's house is completely different, though. Some acquaintances showed up an hour early to my house for lunch once. It completely threw me. I didn't stay mad about it or anything, but I didn't invite them over again, either. It would have been bad enough under ideal circumstances, but my husband was away, I had a new baby (my first, so not much experience), and had planned to nurse her shortly before they were due to arrive. It was a very awkward situation.
Now, if one of my best friends showed up early, I probably wouldn't care, even if it were in my own house. That's because I don't care if they see my house messy, or see me in my housework clothes, or sit in the living room alone while I finish doing my hair.
So in my own rambly way I guess I'm saying that circumstances and the nature of the relationships play a big role in whether or not this is rude. I would say the situation in the OP was probably not, since it was a venue, and the woman knew one of the organizers well.