My wife is in the US on the VWP, and will be leaving the country by bus into Mexico after having arrived by air. I've taken this bus route multiple times, but have no memory of ever stopping on the US side of the border to get an exit stamp - but as a US citizen, I don't need one.
I know that when leaving the country by air, no exit stamp is necessary, because the air carrier communicates their passenger list to the US immigration service.
Is the same thing done on buses?
I can't imagine it is, as lax as the security/ID procedures are on buses.
If not, what must my wife do to ensure that her exit from the US is registered?
Or will her entry stamp into Mexico be enough to prove that she left the US if/when immigration ever asks her about it?