I apologise; I am clearly not familiar with the current state of exceptions. I'm sure it's obvious that IANAL as well, but, outside of specific exceptions (of which salary disclosure is apparently one as of the last decade) at-will still means "any reason or no reason" (though, California isn't exactly the best example of "at-will"....).

Unless my knowledge in this case also happens to be woefully out of date, the burden of proof lies on the allegedly wrongfully terminated employee.