UJM, Your comparisons are not valid. A Reform or Conservative clergyman (I hate to use the word “Rabbi) does not believe that the Torah was given by Hashem and was man made. That is not even Judaism but total Apikorsos.
We all know what the Torah says on the issue confronting this young lady. It is a clear and indisputable Halacha.
The issue is how to explain it to this young girl in a way that she will accept it. Both, the Chabad Rebitzen and the older more experienced one are going to be delivering the same message. The question is are you going to do it like Hillel or like Shamai. Will a “frask in punim” and calling her a shiktza be more effective in convincing her to comply or a non judgemental friendly explanation with a smile on your face accomplish the same goal? I argue that a heavy handed approach might drive her off the derech entirely but a more gentle approach will save a Jewish neshama. Nobody is looking for a lenient psak which is out of the question.