The designation "Jehovah's Witnesses" comes from Isaiah 43:10.
"Ye are my witnesses, saith Jehovah, and my servant whom I have chosen; that ye may know and believe me, and understand that I am HE: before me there was no ùGod formed, neither shall there be after me."
Who says a person is a Witness? Jehovah. The scripture says that He choses who are His witnesses.
Thus, it is not appropriate for an elder to state a certain person is NOT a Jehovah's Witness.
Unless the elder is claiming to speak in the name of Jehovah!