The right thing to do would be this, let her come as she had as always and let the new hires make the change and any existing employees that wished to as well. If the earnings of one vs the other were superior, equal or less than you could say the market was working itself out. geez. way too much goes into that. "here's the new uniform. You don't like it? Work somewhere else". Quitting on her supposed "Christian" principles look better than getting fired because she, personally, didn't like the way the business was changing, including the new uniform.
Now, if the new uniforms cause the business to tank, so be it.
All of you people who say she is wrong to sue are wrong. She started employment there with the expectation of being able to wear the pants and shirts as the uniform. This is an implied contract. The company has no right to change the contract without her approval. The way for the company to handle it would have been to require all new hires to wear the new uniform and the old hires could go either way as they saw fit. Eventually they would have all the girls wearing the new uniforms.
To sum it up. She had a contract, they violated it without her agreement. She will win in court, especially in view of the way they cut her hours vindictively and then dismissed her without cause.