I don't think a PoA is required for the son to submit information.  The CP-2000 unit needs to be told of the 1040-X, which they can probably access even if processing is not completed.  (If there is a balance due, it's not taking six months.)  Is there really a need to speak to IRS?  I would write a letter to the reply address for the CP-2000, explaining the situation.  Have the son sign it, add a sentence that "my mother is incapacitated so I am helping with her finances."  What you are saying is, "We agree we owe even more money than what was reported on the 1040-X," right?  I would also calculate the amount now owed considering both changes, and print out a 1040 with "Information Only -- Do Not Process" across the top of the first page, showing the amount the client expects to owe.  For best results, pay the remaining balance due with your response. 

There is some stuff in Pub 947 on what to do if a Form 2848 is not available, but that is just going to slow down the case.