I imagine if he lived abroad and still managed to owe $20k by his reckoning, his income and/or assets are non-trivial, and the IRS may have some cause to think he owes more (that's not to say they are correct).
Unfortunately the US tax system is a very adversarial one.
A programmer earning around $100k could easily accrue that kind of tax liability over a few years, even after deducting payment of foreign taxes. (But not penalties and interest for the delayed payments)
So he'd probably have less than 50k in income liable to taxation, and that puts him in the 15% bracket, so for 3 years his tax would be about 25k. How they got from there to 140k is an interesting question -- that's a hell of penalty & interest charge.
But you are obligated to file, even if you have zero income, and you are obligated to disclose assets, even if they're your spouse's.
There are several more requirements not related to income, or even if the citizen ever lived in the US (many people have US citizenship and have never stepped foot here), each of which can carry a substantial penalty if violated.
There are many people living in fear -- and many in substantial debt -- not about income tax problems, but about IRS filing requirements. The system is a monstrosity.
Unfortunately the US tax system is a very adversarial one.