That is an extreme, if that is what she feels like doing. Tell her that there is no specific path for enlightenment and such extremes will only harm her body if she is using it as a way to escape from her problems. I've tried extremes and running away from problems too and it doesn't work, what works is facing everything, whether they are problems, feelings or other people.. Personally, i feel devotion and surrender is a beautiful way. Fasting will affect her body more, but with devotion it will affect her heart and her whole being. Suggest that too her, but dont try to force her out of it coz she will only do it more. Let her do it for sometime and suggest her this also, let her change her mind on her own.