hmmmm, this brings a different light to the situation. The work example you gave above, in other words a friend canceling on you, is horse of a different color. When people cancel on me, I don't need to know the reason because it is personal. I am fine if they choose to hide the truth or make up a reason. There maybe numerous reasons as to why a friend cancels on me and I never take it personal because it usually has nothing to do with me. My friend might be going through a relationship crisis, or may need to see a doctor, or may be too depressed to get out of bed...
In all honesty, if I am going to see my doctor, or go to my chemo treatment, or if I am too depressed to get out of bed, I am not going to share the real reason with anyone because the real reason is personal and I am under no obligation to say "i am very depressed. I cannot get out of bed." In fact, instead, I lie and say that I have to work.
So, in summary, you may want to consider that your friend may be going through a difficult time (the reason does not matter) and is not able to share her time and space while struggling with her personal problem. Please try to not take it personally because there is a good chance that it is nothing against you.
Also, one more brief thing to add: every person has a different capacity and definition for friendship and we cannot expect them to fit to our definition of a friend. Every friend is a different treasure and has something else to offer. If we expect them to fit into our own check list, we would end up being very alone (I am not saying that you are doing this. I am saying it because I made the mistakes of expecting too much from my friends. It was my fault). Friendships require flexibility and tolerance as much as they require love and honesty.