Only if they don't hire people because of their religion are they in violation. They hired her and expected her to follow the rules.
Sorry but it doesn't matter why the rule was made. If you tell an employee not to do something and they do it again then they can be fired on the spot.
I worked at one company were talk radio was banned on company property. Some disagreed with this policy. They were warned. Then they were dismissed.
It is no different. She was not being made to do something, she was asked not to do something. If she didn't like it she was free to work elsewhere.
People are spazing over this because the owners are Muslim. And they are being dishonest with themselves because of it. Would you be upset if an employee in a Christian owned company was fired because they hung nude pictures?
bye