Is this a symptom of a certain condition/disease?

terserah - Welcome to Answers. Your friend is lucky to have such a concerned friend as you who cares. As for what caused her to "faint" she might already know and would tell you if you asked her or discussed her fainting with her mother. There could be several causes. One could be a low blood sugar, sometimes ociated with diabetes or with eating very little food, She might have a form of epilepsy known as petit mal where fainting or falling unconscious suddenly and quietly is a happening. Her doctor will likely do some tests and discover the cause and prescribe proper treatment.