Backing out from attending a wedding with 2 weeks notice, if the reason is good, would be forgivable. But to back out of doing what he promised regarding their hair is a terrible thing to do, especially when it's because he procrastinated on this. Chances are, it will be very difficult for them to find someone else on such a short notice.
He should be the one to contact her, and he should do it immediately. This is a very rude thing he's done, close friend or not. He should never have agreed to do this unless he was certain he could get off work. He should also offer to help them find someone else to do their hair, especially if he has any contacts in the area.
You should go on to the wedding, but don't be surprised if the bride gives you a cool reception, too, even though it isn't your fault. As for being mad at him, it's hard not to be, but that doesn't really solve anything. He made the mess. He should face the responsibility for cleaning it up. Keep your anger in check, but make sure he does not dump this on you.