Under the IRS' "People First Initiative" payments were suspended from April 1st through July 15th for those with existing agreements, so it may not be as bad as you think. Has your client received any sort of notice at all?
https://www.irs.gov/newsroom/have-an-installment-agreement-or-payment-plan-payments-should-resume-starting-july-15
https://www.irs.gov/newsroom/irs-unveils-new-people-first-initiative-covid-19-effort-temporarily-adjusts-suspends-key-compliance-program
Has the client set up an online user account? He/she can check the balance, set up the direct debit, revise an existing agreement, etc.... Here's a link to another IRS page in Q&A format with a lot of links that may be helpful too:
https://www.irs.gov/payments/payment-plans-installment-agreements