Third party sick pay not taxable to PA is not an adjustment on the return.  It must be reflected on Line 1a of PA-40 and it is the employer's and/or the agent's responsibility to ensure the correct reporting of PA wages in Box 16 if all the conditions are met.

Third party sick pay, for federal tax reporting purposes, could include payments made by an agent on behalf of the employer on a cost-plus basis but that does not count for purposes of 61 Pa. Code § 101.6(c).

If the employer did not report the correct amount in Box 16, your client should request a W-2C from the employer.  Otherwise, PA DOR generally requires a written statement from the employer and, without which, the taxpayer must attach his/her own explanation of the difference between Box 16 and Line 1a to the return.  In this case, you will simply enter in PTO what Box 16 should be, taking into account the nontaxable third party sick pay.