"She's one of them." is more like saying she is part of a group or a certain category of people. "She's on of those." is like pointing to an object and saying she's one of those. basically putter her as an object instead of a human. Example: if you point to a lamp and say "she's one of those" If you point at a group of people and say "she's one of them" the correct way would be to say "sh's one of them"

How long have you been waiting for a bus? I have already been standing here for fifteen minutes but no bus has come.

hello. Munkh-erdene