They must have a totally incompetent HR director. Unless, they had a termination for cause process in the works before he was called to active duty; even then I would think that process would be suspended pending his return to civilian status. He would naturally have some avenues of appeal. I guess that their business practices are on a par with the quality of their vehicles. My brother bought two of them a while back. I asked him why he would be so foolish. His answer? "They are so cheap it is like buying a disposable lighter instead of a Zippo." Neither vehicle lasted five years.
There might be valid reasons for terminating his employment, such as if his termination was part of a layoff or job elimination affecting more than just himself. Maybe his department was restructured or eliminated. If other, non-reservist employees were terminated at the same time, I don't think he'll have much of a case.
You can't terminate reservists because of the inconvenience of them being called to active duty, just like you can't terminate someone because they were injured on the job, or because they are pregnant, but that doesn't mean you can't legally terminate reservists on active duty or people out on worker's comp, or pregnant employees.