Hi. I think we all have a friend like that. They seem to never carry project to the end and seem to live in the moment with some kind of attention deficit disorder. With that kind of friend, you must assume they are not reliable and accept them like that. If they want to come, tell you will wait for them up to a certain hour and if they are not there, you will carry on without them. That friend will not change, it is in their nature to be like that. She will probably come back to you after a certain cycle.
Sometime the problem is not a deficit disorder but a problem of always putting new things in a routine list and never removing anything back, the result is a major lack of time for everything on the list. In that particular case, someone can change once they realize the problem.
Well, a car does not rest :). Oil and fluid can cool off but that does not take much time. She's driven by dopamine and that seem to be what drive most of her action, not logic. She won't be mad for long. You need to talk to her about things just as you are ready to do them, not in advance because by the time it is ready to happen, she will have drifted to other newer interests. She will always have an interest in you if you can surprise her. You will never be able to rely on her for anythings, you need other friend for that. But, she may be interesting to have around to allow some adventure and unplanned thing in your life (that is how i see that type of friends).