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The only way to contest the subpoena is to ask the court to set it aside. You have not stated what jurisdiction this case is in. That is critical to any answer, laws vary between jurisdictions.
Child support case
Child support case. Can mother's attorney legally subpoena father's girlfriends financial documents to be presented at the support hearing case? If it's legal to ask for, does the girlfriend have to give forth her personal financial information even though she has no involvement in the support between Mother and Father? Father & girlfriend live together and have one minor child. This child. Support case is between mother & father for one minor child.