I just can't understand why someone would purposefully want to ride in an event with those that are faster, if she can't accept being alone for most of the ride. This will breed deep resentment. The fact that she "thought you wouldn't follow through," is just, well, strange.
I get upset when others, all of a sudden, speed up and leave me in the dust. It just reminds me of my weaknesses. But, the people I ride with don't do this. They either ride about the same speed as me, or a little faster, or slower. There are some days DH pushes me to my limit, but I know when those days are; usually I set the limits in the beginning of the ride.
This thread just emphasizes the reason why it's important to find riding partners or groups who have the same goals as you do, or enjoy riding alone. Everyone has expectations of what will happen on a ride, and I find that it goes best when they are clearly delineated before the start.
It's funny, I nordic ski with people who are clearly better than me and it doesn't bother me as much. I feel like I am still working to get there and have seen a lot of progress. For some reason, cycling evokes more "feelings" about this. I say I'm not competitive, but I am in some ways. Not racer competitive, but competitive. I am betting that this is what your friend is feeling. She probably wants to ride like you, but for some reason, can't or won't put in the time.
I ride with a friend who is sort of like that. She makes it clear she will never be as fast as me, and she doesn't even try. My rides with her are just fun and I certainly won't let the friendship die over this. We were friends long before she started riding. I get annoyed occasionally, but then I get over it.