Your legal team will have this well handled, but "case by case" without more isn neither policy nor guidance. Guidance and policy are what are used to determine one case from another. They are what determines "yes" or "no" in each case-by-case.
"Case by case" alone means nothing more than "we will decide on the basis by which we make our decision." And if there is no further guidance on how to decide each case, then it is arbitrary, which would be clearly unlawful.
Guidance says "in this situation, do as follows. In that situation, do this other thing." This is the real policy. And if the real policy says that in this situation, or that situation, a boy is admitted to a girl's facility it is unlawful. If the policy lists factors to be considered when deciding when to admit a boy to girl's facilities - then it's unlawful. It would be unlawful to list any such factors, because there are none that can be taken into account.
OK, they say, but we don't have a written policy. No, maybe not, but there is still a policy, unwritten, unspoken, on the basis of which this case-by-case decision is to be made. It is not done by tossing a coin.