Is this sentence correct ?

[ahmedalsmair]

Ok so today while I was attending my English class the teacher gave us an example, and that is :
I didn't know why he is leaving.
So I stood up and told her that the sentence should have looked like this:
I didn't know why he was leaving.
she told me that both sentences were correct and there were no grammatical mistakes
so the question is are the previous sentences both correct ? If not which one is the correct one ?
thanks in advance and sorry if there were any mistakes.

[Alan]

Strictly speaking, you are correct! However, colloquially, the alternative form is frequently heard.