No 9465. Just file the return (or maybe even an extension) and pay as much as they can. It will take IRS several weeks to process the return. The client can then make payment when they get the notice, or maybe even earlier if they pay online at IRS WebPay

An extension seems to me to be a good choice. They can pay the balance with the actual return when file and have the balance. They will be billed some late pay penalty & interest.