You didn't give details that matter: age of the taxpayer, if this is a retirement plan account through an employer, or a personal retirement account, or what type. Also, you mention he asked for 20% withholding. It's only the tax table next bracket difference that is taxed at 24%. Not his entire taxable income. That's a difference of 4%, if all the distribution falls into the next tax bracket, so perhaps he's been underwithheld all along. You can't really ask for abatement for taxes owed. Perhaps you are asking about penalty? Have you seen this table? You might want to bookmark it:

https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-exceptions-to-tax-o...