>>what can be disclosed, and what kind of client permission is needed<<
Everything, and nothing. Unless the client challenges the subpoena in court, it's impossible for the tax preparer to refuse. Even an attorney can not claim client privilege, because tax returns are intended for a third party (the IRS). An attorney can protect tax planning, but nothing that went into the tax return itself. CPAs and EAs can also protect planning, but only within IRS. The best you can do is charge a reasonable copy fee, like 25 cents a page (and keep their dirty mitts off the originals). So notify your client that you intend to comply by the due date, and tell him to call his lawyer immediately.
Then call your own lawyer or E&O insurance. Divorce---aargh!