eracode:
Do you mean he earns no income in the US (and never has?) but still has to pay US income tax on income earned elsewhere?
This is the nub of FATCA. Non-US banks have to provide information on US citizens to the IRS. Ironically US banks don't have to comply, so it has made domestic tax havens much more popular (several states have "innovated" with their laws to make this easier) - e.g. https://www.theguardian.com/world/2019/nov/14/the-great-american-tax-haven-why-the-super-rich-love-south-dakota-trust-laws