Read Dharma Mittra's response:
One of the main purposes of practicing asana is to strengthen and purify the body. Of course, diet plays in integral part in this. The serious student must follow a healthy vegetarian diet, for health and more importantly for the sake of developing compassion to all living beings—including our selves. The sacred text, the Bhagavad Gita, clearly states, "Yoga is not for him who eats too much nor for him who eats too little. It is not for him, O Arjuna, who sleeps too much nor for him who sleeps too little. For him who is temperate in his food and recreation, temperate in exertion at work, temperate in sleep and waking, yoga puts an end to all sorrows." This is a very important teaching for practitioners of all levels. Constant practice is the key to success in yoga, not going to great extremes. The yogi who goes to extremes in his practice will make slower progress than the yogi who is more moderate and consistent.
For this specific situation, your course of action depends on the student's level of receptivity. If she is really listening and open-minded, it may be enough to speak in general terms to the class she attends. You can say, "If any of you have any difficulties or problems, please feel free to speak with me about them. I am happy to share whatever I may know in order to help the advancement of your practice. I am here to help you to the best of my ability."
If she comes to speak with you, then I would recommend that you have her find the proper suggested weight for her specific height and body frame. Recommend that she establish a healthy vegetarian diet that includes include lots of ripe avocados, brown rice, whole grain breads, nuts, baked potatoes, fresh green juices, oatmeal, fresh salads, and protein shakes. Also, recommend to her a daily Headstand and Shoulderstand practice as well as alternate nasal breathing exercises, which will help increase her appetite.
If your student is less receptive, keep in mind that every person is operating at a different level of consciousness. It may not matter what you say to her; if she is not ready to hear you, there is nothing you can do about it. As with any of the yogic teachings, you can only offer the students what you know, as clearly as you possibly can. What each individual does with that information is up to him or her. As you gain more experience teaching, you must cultivate a deep faith that all your students will find what they need at exactly the perfect time.
It is also important to remember that you, as the teacher, are responsible for keeping all the students safe in your class. If you have a student who is about to seriously hurt himself doing a posture, you are the one who must tell him to break the pose before he breaks his arm! In each situation you must use great discrimination to act responsibly and compassionately. Let yourself be guided by your own divine truth, and from this place you will never go wrong.