My sister-in-law is also "notorious" for showing up late. My mother-in-law doesn't agree with me or any other members of the family about this. I was there once with just my brother-in-law who was making dinner and he asked me what time he should tell his sister to be over. We wanted to eat around 7:00 so we told her 5:00. She showed up right at 7:00. We've told her we are eating Thanksgiving at 2:00 when it's always 4:00 (and she was showing up at 6:00).
But she's family and there's not much we can do about it. Personally, I would have never made plans with these friends if the dinner was that important to me. They are only giving you stress. If you haven't already given them the dinner time, tell them an hour earlier. If you have given them the time, tell them Disney called and it had to be changed to earlier. If they happen to get there on time, tell them why they are early and then walk around the grounds together while they realize they have been rude in the past.