If they're the school rules, and she knows that, then she's at fault. I'm not saying it's not frustrating, or whatever, but it's really not worth making an issue of. She doesn't have to stay at the school if she can't bear to change her hair colour. And if she really wants to stay at the school, then it's not that much of a sacrifice to dye it back.
I went to a private school which had very strict uniform regulations. It was a great school for me, and I felt very lucky to go there. Although I didn't like the uniform much, I pretty much just wore it as we were supposed to. There were always kids who felt angry and resentful about it, but they were the ones that just kept breaking the rules, even though they were very clear. It was like those kids were above everyone else, and thought there should be a separate set of expectations for them, because they were so special.
Frankly, I think it's a spoilt way to act. If you're lucky enough that your parents can afford to send you to a private/Catholic school, then be grateful for it, or tell them it's not your thing and go somewhere else. All schools have pros and cons, nowhere's going to be perfect. In some ways, I would've loved to go to a school where I could wear Converse, and shorts, (we always had to wear dresses/skirts) and where we didn't have to go to Chapel services once a week. But there were things that I really liked about my school, so in the end, it didn't matter.
I think this girl is making a big deal out of nothing, and should either stick to the rules or go somewhere else. Any sort of system, whether it be school, uni, work, or society, has certain expectations. When these are as clear as school rules, by attending a certain school, one would assume that they were implicitly agreeing to those rules by becoming a part of that system.