Because of the Dramatist's Guild Contract, he actually does have the right to fire anyone he wants (including actors and orchestra personnel). The Guild contract prevents anyone tampering with so much as a word of the script without his approval, as well as the right of approval over the director and all personnel -- he has final word on all casting and other hires and he has the right to withdraw his score at no penalty if he doesn't get his way (this covers playwrights, composers and lyricists).
Still, a pretty sh*tty thing to do considering the circumstances. He's generally rather easy to deal with (though, he can be a little prickly, so it probably wasn't a good idea to approach him if you didn't even know how to say his name properly).
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney