One method is to just start paying. No plan, just pay.

Client can make payments online or by mail. Client gets and IRS notice of amount due, send a payment in response to mail. Client does not get an IRS notice, send a payment. Regardless of payment plan or not, there are penalties and interest, so paying as much as they can, as fast as they can is the best approach. 

When the get a CP504 then it is definitely time for them, or a representative to make direct contact with IRS to discuss a plan. 

Tell them to NOT respond to any e-mail or phone calls from the outfits that will help them resolve their issues. They get big bucks up front (out of fear) and generally don't do anything you or the client can not do. 

If you don't feel comfy doing this, help them find a local Enrolled Agent to help them get it done. 

P.S. Get your prep fee before delivery of the return.