A question is why he was leaving "within a week." If he was planning to quit anyway, and the company simply stepped up the timing BECAUSE of his exercise of protected speech, they might be liable only for a week of pay, etc.
You do not have to be a member of a union to be protected. Talking with fellow employees about workplace issues is generally protected because it is acting in concert. Another question is whether he violated a company policy in using email or some other mechanism, such that his process was not protected.
http://en.wikipedia.org/wiki/National_Labor_Relations_ActBut given that the NLRB recently held that Facebook posts can be considered protected speech, maybe he has a case.
http://www.huffingtonpost.com/2011/05/24/facebook-posting-worker-fired_n_866353.htmlCaveat: I am not an attorney, but these questions are related to the job I do have.