My son, a US citizen, lives and works in Germany. Last year he married a woman who is a German citizen. All of his wife's income is foreign earned.
99.9% of my son's income is foreign earned.

Does he have to list her income on his US tax forms?...if so, which form would he use?

I understand that because she is not a US citizen and none of her income is from any US source, Uncle Sam cannot tax it. Is that correct?