Can a boss fire someone for what they wear to work? Even if the were not fore-warned and this happened once?

I used to work in a mailroom bank environment where the dress was casual. One co-worker had been on vacation in the Bahamas and when he returned after 3 weeks, he came back to work on a Monday wearing Bahama shorts, sunglasses and a t shirt. My boss saw him and got mad, stating that if he wanted to dress like he was on vacation still, that he could take a permanent one, and fired him on the spot. The co-worker stood speechless as the boss walked off pissed. The co-worker usually dressed casual and probably never thought the boss would fire him for coming to work like that. The boss never fore-warned him and this was never an issue before. Anyways, I left that place before I could really find out what happened. Can boss really do this though?