Hello Galaxy,
According to the grammar rule, "that" cannot be used in non-defining clause because "that" can be used only in defing clause to replace using "who/whom/which" merely for an informal use. Examples: I know a man who is very good at playing piano. (formal)
-->I know a man that is very good at playing piano. (informal)
As u can realize, speaking is about confidence and communication, and we cannot really think quickly what is formal to say or whether we should use "who/whom or which". Morever, speaking is informal, I guess. So, then we say out quickly and that way we use "that" in order to be easy to say what we want to. Please remember that in writing using "that" is rarely seen or used. Talking about non-defining clause, we use relative pronouns like "who/whom/which/whose" between two commas, dashes or parenthesis or with just one comma/dash/parenthese when the non-defining clause stands at the end of a sentence. Please see examples below.
Examples: Mr. Paul, who is a great teacher, is wise. (right)
--> Mr. Paul, that is a great teacher, is wise. (wrong)
+I am going to go to Angkor Thom bookstore, which is near your house.
Thus, please note that "that" cannot be used in the non-defining clause because it is for an informal use in the defining clause. You know, the non-defining clause is a very formal one and seen mostly in a formal writing.
Enjoy yourself!
I'm glad to be of some assistance.
