Your client can go to IRS Direct Pay and make the initial payment, then complete the 9465 application online, Id have them wait until IRS has the return processed and the balance due is updated in the computers before doing this

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

...you cant do anything electronically from your end once the return has already been filed....you could fill out the paper application and they could mail it in, but it will be sitting in a pile for who knows how long before someone opens and processes it.