What if the client has a W-2 but not enough withholding?

To obtain an Offer-in-Compromise with the IRS the taxpayer must be in tax compliance during the Offer and maintain compliance for the five years following acceptance of the Offer. If the taxpayer is an employee and is not having sufficient withholding done to avoid incurring a tax liability at the end of the year, then they are not in tax compliance.
The taxpayer should fix their withholding and submit proof the withholding has been corrected with their Offer. They should also make an estimated tax payment to make up the shortfall. If they cannot or will not, then they are not in tax compliance, and the Offer will be rejected as non-processable for lack of compliance.