Has form 56 been submitted to the IRS? This simplifies the process as a 2848 can be used instead of a non-IRS POA which has to state specifically what year and tax form it applies to and contain wording used in the 2848. After that, if the return is e-filed, the POA, or the 2848, has to be mailed in with form 8453.
http://www.mclr.net/siteAssets/site12603/files/MCLR_Signing_a_Tax_Return_for_Someone_Else.pdf
https://www.irs.gov/instructions/i2848