first of all noone decides to fall in love, it just happens. and Arohi loving Arjun was in no way cheating on her best friend in another way since Arjun never loved Gauri and also Arohi thought her to have moved on as she was married now.
and by now, it must have been clear to all viewers that Gauri never loved Arjun, she is just obsessed with him. And u know Arohi too never thought Gauri to be in love with Arjun, but just infatuated.