A US citizen (A) marries a G-4 visa holder (B) who lives in the US. They elect to treat B as a resident alien for income tax purposes so they can file a joint tax return.
You are a U.S. citizen (even if you reside outside the United States) or other U.S. person (including a resident alien individual). Instead, use Form W-9 to document your status as a U.S. person.
Does the election mean that B should file W-9 instead?