Thanks to ProspectiveBum I found the $600,000. But, I still can't find support for it in the article, and that still looked awfully high, so I went looking.
According to the Fed, outstanding consumer credit in Sept 2014 was:
$ 849 billion, "Revolving"
$ 944 billion, Auto
$1,311 billion, Student loans
$ 144 billion, other non-revolving
----------------------------
$3,248 billion, Total
If I assume auto and student loan loans charge 6%, and credit card and others charge 13%, I get total interest paid as $264 billion.
There are about 230 million Americans over 19, so this averages $1,150 per person per year on $14,000 of debt.
If people pay that for a full 65 year (to age 85), that's $75,000 lifetime interest payments on non-mortgage debt.
According to the Consumer Expenditure Survey, household that are paying mortgage interest averaged $8,100 in 2013.
About 41% of adults lived in households that were paying mortgage interest (the rest were renters or lived in paid-for homes).
If people paid $8,100 per year for 30 years, that would be $243,000 lifetime.
But, most people who buy houses are married when they are buying, so let's say $150,000 lifetime mortgage interest payments per person.
So I get $225,000 lifetime for the average American.
That's a lot of money, but also a lot less than $600,000.