Well, there's no doubt that he owes the money regardless of not knowing about it.

Here are your options:

1 - Pay it.

2 - If you have the cash, offer a smaller amount. They might take it but make sure you get a binding settlement agreement in writing before sending the money.

3 - Ask for a payment plan. You might get it, you might not. They have no obligation to give you one.

4 - Obviously, if you don't have the money, you can't pay it and you'll suffer whatever consequences come along for not paying.

But if your real question is how do you get rid of the debt without paying it at all, that's not going to happen.