The best thing to do is to pay as much as he/she could afford.  Depending on the amount owed, perhaps consider applying for a payment plan but interest and late payment penalties will continue to accrue on any balance due.  This may, therefore, not be the most cost effective option if your client has other means to finance the bill: https://www.irs.gov/payments/online-payment-agreement-application

States also have similar offers but may condition the plan on the taxpayer experiencing genuine financial difficulties.