You don't ever need should you ever tell a client about anything other than your policy. In other words, don't tell them what kind of a bind, or this is how you make a living etc. It lands on deaf ears unless that client is also in the service industry.
But she is a fairly regular client and while I don't agree with the hour before appointment has she done it before? If no, then this is her one chance to do so and for the most part she will get a 'by'. Until you do a formal cancellation policy you have to be icy professional about it. Or until you are booked solid and your clients know if they cancel they won't be able to get back in.
In the meantime: I am busy crafting a policy I will post and y'all can do with it what you will.