I have to agree with the majority here. If I was the student and no show no called I would expect to get charged because that is disrespectful. If I got last minute tickets to a sporting event that I wanted to go to and called to cancel I would still expect to get charged because I decided to go to the event instead.
On the other hand as mentioned if something came up at work or a family emergency came up I would notify my instructor and would not expect to get charged knowing I cant control these types of accurences.
If I was the instructor and the student cancelled last minute on a regular basis regardless of the reason I would no long keep them as a student.
I think you should use your best judgement, if you think this was something they couldnt control or that they will be back several times throw them a bone and dont charge them. If it is the 3rd or 4th time charge them and then decide from there if you want to keep them as a student.