I’m not a lawyer, but how does someone subpoena documents? Is she prosecutor?
I’m not a lawyer either — I believe that as part of discovery for her suit, her lawyer would ask the judge to issue the subpoena.
In order to subpoena documents you need to have subpoena power, as specified by law, or by private agreement (such as in a private arbitration). Generally it arises out of a particular kind of legal or administrative proceeding. You do not have to be a judge or government official to issue one, but there has to be some kind of pre-existing legal process that it relates to.
Attorneys for private litigants can issue subpoenas, but there has to be a lawsuit pending first. The person who is subpoenaed can ignore the subpoena, in which case the issuer will have to move to compel compliance, or the person subpoenaed can move for a protective order to get the judge to block it, wholly or partially.
Certain government boards of inquiry, Congressional investigatory bodies, and private litigants in private arbitration can also issue subpoenas, in regard to an ongoing proceeding.
But even if you are the kind of person who can issue a subpoena, you can’t do it out of the blue. There has to be some kind of proceeding that it relates to.