Bottom line is that a Christian is a Christian - regardless of the cult's opinions!
Here's a hypothetical scenario which I've rehearsed in my mind and plan to use if/when I meet my still-in friend:
Me: If I asked you what your religion is, what would you say?
J.W. friend: Why, Jehovah's Witness of course!
Me: No, your religion is Christianity. Hence the reason why you're a Christian! Your denomination is J.W.
J.W. friend: Mumble, mumble, mumble. (can't deny the terms Christian & Christianity)
Me: So if I asked you tomorrow, and the next day, and the next day, what your religion is, how would you answer?
J.W. friend: (reluctantly) Christian I suppose.
If it occurs, I will post the actual response.