My friend complains about having severe headache at times, may be 5 times in a month and the pain is unbearable to her. What could this possibly be?

Exam and history. Too vague a question without having a thorough history and an examination. Too many variables when attempting to determine the cause of a headache.
Possibly migraines. Migraine headache is quite common and your friend could possibly be suffering from it. It is often quite severe and may worsen over time. She should see a doc for an evaluation, esecially if this is a new headache or if it is rapidly worsening. Excedrine migraine may help some people minimize the symptoms. But she may need preventive treatment. Wish her some luck for me..