Is sexting someone else during a long distance relationship cheating?

Never mind the technicalities of what to call it. It is wrong, and she knows it is. And if I were her boyfriend and found out about it, I would figure I had been mistaken about her character and break up with her. If she wants to stop it, why can't she just tell the other guy that she has begun to feel uncomfortable about it and would rather not keep in touch with him any more. That is, "break up" with HIM.