Strictly by coincidence, CBE Group is located in Iowa, whose Senator Grassley (age 87) is chair of the Senate Finance Committee, which has IRS oversight.  I'm sure an Iowa voter could solve this problem with little effort, by a phone call to one of the Senator's local offices.  Not sure about the rest of the country.  

IRS might have suppressed those notices, and not told its collection agencies about it.  How much did your client owe for 2016?  I thought these accounts that were being farmed out (as they say in Sioux City) were mostly small balances.  So small, that your client didn't even notice it was not paid.