Being trans does not give extra dress-code rights, and nor should it. None of the other women are allowed to dress that way, so why should she?

Now, if she wants to challenge the dress code for more esoteric modes to be allowed, that should be taken under consideration by whoever is in charge of that, but in the meantime, she should at least try to conform. Then if the decision was to go against her, she'd have the requisite conforming clothing already.

(Tangentially, there's an argument that gender non-conforming people might want to define other professional dress codes that don't strictly fit with male and female norms, but that's doesn't seem to be what's happening here.)

I understand that it's difficult for trans folk who deal with transphobics everywhere they turn and thus every discrimination could be transphobia, but this one seems pretty easy to test.

And I have to wonder how she'd react if she won the dress code change and other people, cis people, started dressing more like her.