AE - they can keep him on as an elder because the organization allows them to do so.
GG - I can only answer your question from what I knew before - any of the current elders on the board might have a more accurate answer. Typically the elders would want her to study again with someone they consider spiritually strong. Who she wants to study with her is irrelevant, by the way. This is because the "truth" is constantly changing so what she was taught all those years ago is more than likely, not the current teachings. Unless she was d/f'd or d/a'd, I don't think they can keep her from the ministry, although they will probably assign her a partner at first. They will probably also want to ascertain her current status (i.e. has she re-married, b/f, etc.) And they could, in fact, disfellowship her NOW that she's going back if they feel she committed adultery in the past and hid it and was not properly punished for it.
Each case is different because of the circumstances.