Working at Coke and drinking Pepsi

[steelcmg]

So I have a friend who works at coke and he told me that if he gets caught drinking Pepsi (or any other of cokes competitors). While he is working he would be fired. I’m not sure on how this works but is this legal? He wants to quit soon so I told him to test the waters and then talk to a lawyer about it. Should he try this or will he lose the case?