Hello, there are two questions as below
1. They are the ones who ____ tried on the costumes yet.
2. I am the one who ____ tried on the costume yet.
My answer for the two questions are the same --- haven't
But one of my teaching partner (from USA), he told me the answer for 2nd question is "hasn't", with no doubt.
I ask him about the reason, then he just told me that English is his mother language, he knows it well.
He was so sure the no one would use "haven't" in the 2nd question.
I'm really confused, the answer "hasn't" is against the grammar I learned before,
and he was so sure that using "haven't" is totally wrong.

If the answer got to be "hasn't",
my explanation is "who hasn't tried on the costume yet" describe "the one", not " I " in 2nd question.
Am I right? and why can't I use "haven't" in the 2nd question?
Thank you all in advance.