What does the Supervisors boss have to say about it? Generally a supervisor doesn't have that ability in most companies. Did he go in all hot and bothered?, has he been told before what his work hours are? generally for a person to get fired on a first offense that almost always is a major faux pas on their part, like fighting, racial epithets, major safety violation, major policy violation.
A chat with an employment lawyer might be a good thing, public policy issues may apply in that state.
Firing is a big deal for both parties, it can be the kiss of death for a professional, especially where they may not be all that many jobs for that profession, even in good times. Plus the word is going to get out that he was fired. Plus on the pragmatic side for a company, they now have to replace a trained employee. In a big company with a even streamlined HR process, that can take up to 2 months, even more if they have to do much of any kind of a search and/or relocation. Plus others have to do his job and theirs, etc
I don't think you got quite the whole story.