The "other woman" implies that there is a relationship going on with two women and that man is cheating - on BOTH of them.
That's a bit of an assumption. Presumably the girl cheating with the cheating guy gives implicit consent for the cheating guy to cheat with his wife/girfriend.
Then again, maybe it's an open marriage and the cheating guy is in fact not cheating on his wife, but cheating in the other womans mind, so as to make it all the more titillating for her...who then is he really cheating?
The question and nickname do say a bit though. I'm guessing she can't decide exactly which status to accede herself, even though she knows the technical tag, she doesn't feel it matches in her own mind...never mind that which tag to attribute wouldn't generate such questions had she heard it from a friend before the current experience.