Client resides in TX, works remotely in GA, never goes there. Employer withheld GA taxes. PS is saying all income earned from the Georgia employer is taxable, is it?

Maybe...youre filing a non resident GA return?    Generally your resident state would give you credit for taxes paid to the non resident state, since the resident state would tax the income as well....but since TX has no state taxes, I guess you only pay the non resident taxes.

"works remotely in GA, never goes there"

is a contradiction in terms. If he never went to GA and performed all his services in TX, the income is not taxable in GA. He didn't fill out the proper paperwork with employer.

GA is not one of the states that tax nonresident telecommuters.

GA employers are, however, required to withhold on wages paid to nonresidents for services rendered in GA if more than 5% or $5,000 of their wages are attributable to GA.  At the end of the day, however, as Susan explains, only GA-source wages (generally based on services performed within the state of GA) should be taxable.  Your client should make sure the proper withholding certificate is submitted to the GA employer to prevent this from recurring this year.