(1) I called out to the student who was turning around to see me.
(2) I called out to the student, who was turning around to see me
I understand the comma before who makes a difference in the meanings of the two sentences. I also understand that the relative pronoun who can be replaced with the relative pronoun "that" in sentence (1).
But what about the relative pronoun who with a comma? Can it be replaced with the relative pronoun that? I think it cannot be replaced.
Could you help me clarify it? Thank you always.