And I am sure that is your friend's internal justification for her behavior:
1. I am a very good friend, very loyal and understanding.
2. I have a very stressful job which can make demands of my time at unexpected moments.
3. I had to drop out of this event because of my job, and my friends (who I have stood beside in the past) won't cut me a break.
Except, it wasn't this event. It is every event. Friends do cut each other a break, but when it is every time, and the same flaky behavior, even the best friend will start to get annoyed.