The measure is what would be considered defamatory (and/or character assassination) by the typical reasonable citizen.
Saying “So-and-so is no longer one of Jehovah’s Witnesses” is no more offensive to the typical reasonable citizen than saying “So-and-so is no longer a member of the First Methodist Church”.
Because the typical reasonable citizen does not associate such language with defamation then it is safer in legal terms.
However it could be argued that JW's have a closed community with its own language and sub-culture, where saying “So-and-so is no longer one of Jehovah’s Witnesses” is equivalent to saing “So-and-so is is immoral, shouldn't be trusted, and is a low-life” to a reasonable citizen.