A fully executed form 2848 is a "power of attorney," specifically for income taxes. You don't have to also send the IRS any other power of attorney form the taxpayer may have completed for other reasons.
If you can't get the taxpayer to sign and execute part 1 of form 2848, then you need to sign part 2 of form 2848 (declaration of representative) and also attach a copy of the other POA form that the taxpayer executed (a state form, prepared by an attorney, etc.)
