As yoga teachers, it is important that we remind our students to use powerful words carefully. Each word carries a vibration, and when the word is abused or overused, it becomes hackneyed and loses its power. Thus it is with "enlightenment."
One can move into and out of states of consciousness, but never out of enlightenment. Enlightenment is a permanent state, and a great deal of respect is due to someone who has reached that level. Everybody claims to be enlightened nowadays, even when they've had a simple spiritual experience. Even the most ordinary kinds of metaphysical experiences seem to engender books, DVDs, T-shirts, and fame.
To be able to answer your question, I need details about what your student is experiencing. Is it a state of heat? Of not knowing where she is in the physical body? Of knowing everything there is? Of becoming completely thoughtless and entering a state of absolute bliss? Is it a state of samadhi, where she becomes one with her true Self or the Universal Source? The next thing to know is why she feels frightened. Is it because she doesn't know what she is experiencing, or is she intimidated by the power of the experience itself?
As you can see from these questions, you, as a teacher, need to cultivate your own awareness enough to be able to help her. Whenever a student experiences something that I have not experienced, I recognize that I cannot advise the student. In those cases, I refer the student to a teacher who can, and that is my suggestion to you.
Recognized as one of the world's top yoga teachers, Aadil Palkhivala began studying yoga at the age of seven with B.K.S. Iyengar and was introduced to Sri Aurobindo's yoga three years later. He received the Advanced Yoga Teacher's Certificate at the age of 22 and is the founder-director of internationally-renowned Yoga Centers in Bellevue, Washington. Aadil is the director of the College of Purna Yoga, a 1,700 hour Washington-state licensed and certified teacher training program. He is also a federally certified naturopath, a certified <a href="/health/ayurveda">Ayurvedic health science practitioner, a clinical hypnotherapist, a certified shiatsu and Swedish bodywork therapist, a lawyer, and an internationally sponsored public speaker on the mind-body-energy connection.