To the threadstarter/Phoebe, first of all we are sorry to hear about your situation. It must had been a shocker when this thing happened, all the 30 performers must had expected a nice photo of them taken when they are in action, but all of them must had been very disappointed, maybe even sad or angry. You probably had a hard time to answer to your boss, your peers, and your guests and performers.
I took a look at the attached SMS and felt that a hiring contract had taken place. When he made an offer to you, you accepted, its sealed and documented in the SMS thread. You made changes to the timing, he agreed, again reaffirming his taking of the job. You proceeded to brief him on the requirement, he acknowledged, further affirming it. There is no further changes brought up so it should remain status quote. But he did not turn up, rather casually, and he caused unrest and unhappiness to the event. Doesn't seems like he gives a hoot about it. He is legally liable if you like to pursue any lost, but dont take my word for it, coz the compensation is unknown and subject to many factors. May not be worth pursuing in court. Consult your legal people.
I also feel that you had extended a nice gesture to provide photo credits and had been straightforward in accepting his charges without trying to cut his rates or reduce his income. The common (and insultingly sad!) market rate for amatuers here is "free" or "you can eat the buffet" or "$25 for 2 hours". So, all the more I feel bad for you when you and your organization were victimized by unprofessionalism when on your part you were pretty steady and the information were clear IMHO.
Unfortunately, you were hiring at the wrong place. Clubsnap is a hobby forum filled with all sorts of people. Anyone and everyone is claiming the title of photographer. Some amatuers are great part timers, yes thats a fact, but also an extreme high number of amatuers are unknown persons of varying behavior and skill levels. I am sorry to tell you that a events photography pro's rates is actually not too far off this hobby person's rate. $180 per hour is common, so you are only paying $50 more per hour for peace of mind, better experience and better photos.
When you went with a amatuer hobby person, he may be treating it as a hobby and does not take it seriously. Why would you want to do that for an important event involving many important people to answer to? I cannot understand the reason why you did went with a professional photographer. He is definitely in the wrong but you are also partly responsible too for poor decision made during the hiring process. Sad to say, but true.