Rpm,
He could vote for whoever candidate he wants. He offended one of his company's clients. His company has every right to fire him.
OTOH, if he were to merely state to his employer that he was voting for Kerry and then got fired, that would be wrong.
I would state the same if the roles were reversed as far as candidates.
As an employee, you have a responsibility to control yourself in a professional manner whenever you're representing your company.