Even if it loses 90% of its value it's still a win if the money came from credit cards you never paid back. Although the couple of hundred bucks you get might not be worth the hassle
You will also be unable to take out any debt upon release. It's difficult to get credit as a released felon without a history of defaulting on debt.
So, being generous and saying that he sees a 4% return on his investment, he'll have slightly more than he went in owing, and will be completely unable to even get a car loan, a decent job, or place to live.
He will have turned 0 dollars in to a lifetime of hardship.
If all of the defaults occurred >7 years ago and there's been no credit activity since the individual would have no credit score because of no credit history. I know this because I've literally seen it happen. What you do then is apply for a secured credit card (basically you pony up some cash to secure an extremely low credit limit) then after you've established a payment history and a credit score you can move on from there. As far as I know financial institutions do not take into account one's felon status when determining elgibility