Trump will get an angry expression at his face, become silent, and call a meeting with his financial advisors the next day. Seeing as exports to the US is 1/3 of the mexican economy. Trump will use this to his advantage and slap a 500 percent tariff on mexican goods.
This will cause mass unemployment, economic recession, a crime wave, and a meltdown of Public spending in Mexico.
When the mexican president goes on Public Television and asks the UN and IMF for short term crisis loans and medical supplies. Trump will return to the person who said "Mexico is the greatest country in the world" and ask him "What do you think of Mexico now, bitch!"