L.G.
People who are chronically late often have a tremendous fear of being stood up or being too early and having to wait for someone while they are all alone in a public place (bar, cafe, etc). Also, there is a fear that they will be too early and consequently have wasted time that they could have used accomplishing something else (darn! I left 15 minutes early and now have to wait when I could have left my house later and finished making the beds). Weird, right?
I got better when I had to be on time for work-related meetings and events and knew that being late would impact my career. Having two little children also helped in a way because although we were sometimes late, I also just slowed down and knew that I couldn't make up time by rushing into the car, speeding on the highway, or running up the stairs. Since my babies would force me to go slowly, I had to build in that time to my schedule and just accept that it now takes 30 minutes to get into the car.
I don't think you can change someone in this area though. You can point out that you understand she may be worried about having to wait in a public place. You can point out that it is a little hypocritical to expect others to wait when she doesn't want to wait and that it is a bit selfish. Ultimately, though, this change has to come from within.