Since when do countries extradite for debts? I'm pretty sure you have to commit a crime, and the crime must be something that is a crime in the extraditing country as well.
You are probably right, although if it is a government student loan, emigrating with the sole purpose of avoiding payment could be construed as fraud.
They could also bring a lawsuit for the debt in the country where the US person emigrated to. This is much harder.
You have any examples where this actually happened?
They won't extradite for student loans. For someone that far in debt, leaving is likely the best option for them. They can still be sued though and I'm not sure how that works exactly when you're in different countries.
If the amount is large enough they hire a foriegn lawyer and bring a lawsuit in the country of the ex-patriate.