Is 'ever' correctly used in the sentence?

[Tan Elaine]

"The crisis we're facing today is the most critical natural disaster that ever happened in Thai history," Thai Prime Minister Yingluck Shinawatra told reporters yesterday.

Is 'ever' correctly used in the above sentence? If so, why is it correct although it is an affirmative utterance?

Thanks.

[bhaisahab]

"The crisis we're facing today is the most critical natural disaster that ever happened in Thai history," Thai Prime Minister Yingluck Shinawatra told reporters yesterday.

Is 'ever' correctly used in the above sentence? If so, why is it correct although it is an affirmative utterance?

Thanks.

What would you replace it with?

[Tan Elaine]

What would you replace it with?

Thanks, Bhaisahab.

"The crisis we're facing today is the most critical natural disaster that ever happened in Thai history," Thai Prime Minister Yingluck Shinawatra told reporters yesterday.

I would replace 'ever' with 'has'.

"The crisis we're facing today is the most critical natural disaster that has happened in Thai history," Thai Prime Minister Yingluck Shinawatra told reporters yesterday.

[Raymott]

Thanks, Bhaisahab.

"The crisis we're facing today is the most critical natural disaster that ever happened in Thai history," Thai Prime Minister Yingluck Shinawatra told reporters yesterday.

I would replace 'ever' with 'has'.

"The crisis we're facing today is the most critical natural disaster that has happened in Thai history," Thai Prime Minister Yingluck Shinawatra told reporters yesterday.

I agree with adding 'has', but I'd keep the 'ever'.
The present perfect is indicated, since the disaster is happening now.
'Ever' is not strictly necessary, but it's probably most common in this sort of construction.
He could also have said, "... the most critical natural disaster ever to have happened ..."