Who qualified him?
But I was speaking of the hypothetical case where a person was elected and took office, but then was found to not be eligible.
Under US Law and the US Constitution, there are four required steps to qualifying a president: (1) registered voters vote for Electors in the general election (2) a candidate receives a majority of the Electoral College votes; (3) a candidate is certified by the President of the Senate (the Vice President) at a joint session of Congress as the person who received the most Electoral votes. There is a process in the US Code of Laws to object to the certification of the Electoral votes of an ineligible candidate or an ineligible elector. And (4) the certified winner is sworn in on Inauguration Day.
The Constitution and US Law is mute on any other process except impeachment, trial and conviction to remove a person from the presidency once they assume the office.
An ineligible person in the office of president can be tried in the Senate for high crimes and misdemeanors and removed by the votes of 67 Senators.
Who qualified him? The Electoral College when they awarded him the 365 electoral votes he won in the November election and the Senate when they certified the results.
But I was speaking of the hypothetical case where a person was elected and took office, but then was found to not be eligible.
In the case of the presidency, that person is impeached and removed from office, and the vice-president becomes president.