If this is helpful, here are the rules that govern non-IRS POAs and what IRS requires in order for them to be acceptable, and how to perfect a non-IRS POA. It comes from Pub 947 but has the code sec cite of when the person with authority may sign a return and those requirements:
Processing a non-IRS power of attorney.
The IRS has a centralized computer database system called the CAF system. This system contains information on the authority of taxpayer representatives. Generally, when you submit a power of attorney document to the IRS, it is processed for inclusion on the CAF system. Entry of your power of attorney on the CAF system enables IRS personnel, who do not have a copy of your power of attorney, to verify the authority of your representative by accessing the CAF. It also enables the IRS to automatically send copies of notices and other IRS communications to your representative if you specify that your representative should receive those communications.
You can have your non-IRS power of attorney entered on the CAF system by attaching it to a completed Form 2848 and submitting it to the IRS. Your signature is not required; however, your attorney-in-fact must sign the Declaration of Representative (see Part II of Form 2848).