Client cannot pay their tax bill

That is pretty much what I do.

Tell them to minimize penalties and interest, pay as much as they can by the due date.

I usually tell them if they are able to pay off State, do that and then they only need to deal with one tax agency for what they owe.

Let the client know what the IRS will tell them:  If they can pay it within 4 months (120 days), the IRS will pretty much just say "okay".  

If it will take them longer than that, they should set up an Installment Agreement and then send them the links for that.  The lowest cost is to set it up online, and with automatic withdrawals from their bank account.

https://www.irs.gov/payments/payment-plans-installment-agreements

https://www.irs.gov/payments/online-payment-agreement-application

Especially now, IRS will likely be very slow to contact tax payer. As Bill said - try to pay of the state, they tend to be more aggressive. An installment agreement costs some money to set up. If the taxpayer feels they can pay the amount of in a year or so, have them just start sending money on a regular basis - putting a note on the check "2019 Form 1040 SSN- xxx-xx-xxxx" and mail to the same address as the regular mailing or even better use https://www.irs.gov/payments