What Causes Sudden Fainting With Lips Turning Blue?

So my friend "fainted" (for lack of a better word) yesterday while visiting me at my work place. Her eyes were open and shortly after she fainted her eyes began rolling back in her head. Her lips turned blue and then quickly turned white. Her body tensed up and then relaxed and after about 10 seconds she sat up like nothing happened. She said this has happened before but seemed to shrug it off like it was nothing. She has not told her parents or her doctor. What could be the problem and how severe could it be?