Options :

1) File the returns without paying; the IRS & CA will shortly send letters   - "HEY, where's the money"?

At that stage, the authorities can be contacted to set up an plan for installment payments.

2) File an extension showing the amounts due, but don't send any money (or, send some money?).    THEN, the letters from the IRS & CA don't show up until after the returns are actually filed.  Actually, the CA extension is automatic if no money is being sent ** .

HOWEVER @Dusty2 is correct, late payment penalties and interest continue to accrue until full payment is made.  Make sure your client understands that 'minor' detail.

It is 'critical' to either file the return, or the extensions so that late filing penalties are off the table.

**Personally, I would send some $$$ to CA, to prove the extension.... I don't like relying on that 'automatic if no money sent' clause.  But I do have OCD issues 😉